I tried MM5 now and thought I'd do a breakdown of the reasons this is still the case, so I can get some feedback on if things are going to change or alternatives to what I'm doing/complaining about.
By far my biggest issue is with adding new files to the library. I'll get to those in the end and start listing the smaller issues. Mostly these are small issues that don't really affect my decision on MM4 vs MM5, but they do detract from the overall MM5 experience.
Installing (got the latest, as of this date, test build, non-debug):
1- After installing MM5 and auto-importing the stuff from MM4, a bunch of nodes which I have explicitly disabled in MM4 are active in MM5, like the "Web" node. Small enough issue that's quickly fixed and, to be fair, every new install in MM4 would also turn it back on. But it would be nice if I could keep it disabled.
2- Global hotkeys aren't imported. I have to set them up again. Once more, small issue quickly fixed but would be nice not to have to worry about it.
3- Other small options aren't imported as well. I noticed it on "Default 'Play Now' action", playlist selection on auto-DJ (it defaulted to entire library) as well as a few other settings. Same as before, small issue, would be nice to have

4- It didn't import the current list of songs from MM4 from Now Playing.
Overall, not much issue with installing, just some small things which would have been nice.
Usage experience:
5- Tried to make the layout similar to what I had in MM4, failed. Basically, I was trying to have the media tree and album art/preview on the left sidebar, which I succeeded in doing. But I wanted to see the current Now Playing list. Instead, I have a big album art preview (cropped) and the album art/preview on the left. Which is redundant.
Most I could do was to add the playing list to the right side, which is not what I want. I need to see the whole list, with a bunch of selected fields, like on MM4. And I found no way to do this. I can actually configure the right sidebar to do what I want, but I'll still have most of the screen occupied by the redundant middle panel, which I can't get rid of. I can't even stretch this sidebar to use most of the screen. I changed skins and it allowed me to stretch the sidebar, but still can't get rid of the other one.
6- Node travelling doesn't actually work as you expect it to. I expand the artist node and I type the band name, but instead of navigating to the band node, like MM4 used to do (and you expect windows navigation to do), it instead inserts the typed characters on search and you then have to double click the artist node to have it actually selected. This is a small issue and I could probably get used to it, also I'm sure it's more of a personal preference, but I thought I should note it. Maybe add a setting for what behaviour one should use?
7-Track selection: I had already noticed this last time I tried MM5 and it remains the same. Sometimes I want to find an artist and play the songs marked with 5*. This was easy to do in MM4 thanks to column browser (although column browser will sometimes disable itself randomly for some reason). The way it is now, I select an artist and have no way to easily filter results I want, like I did with MM4. All I can filter is which albums I want and then I have to manually select files. This is a hassle for large collections. I just want to mostly select an artist and be able to filter their songs by either rating or genre, mostly the first one.
8-Track selection 2: Giving a real example- I have the remastered editions of the first Marillion albums. I want to select a few among the first 3-4 but play only the first disc. I select those albums from the album list so I can then filter the ones I want but the track list isn't updated. I still have the whole discography selected on the track list. Or I select one album, same thing happens. With just one album I can double click it and it will show me the tracks for it (travels to album node, even though it doesn't expand the albums node itself), but that doesn't work for multiple albums.
9- Multiple tracks selection. If I drag from an empty space below the list to select however many tracks I want, it won't work. Instead I have to start dragging on the file itself, being careful not to hit any text, otherwise it behaves differently. Small issue, could get used to it, but would be nice if it worked like expected, even though MM5 is web based not windows based.
10- The library only has an artist node. I have many compilations (album artist is Various and Artist is whoever plays the track) so I tend to navigate album artist a lot more than artist. In fact, the latter one is quite rare.
11- Player only allows showing remaining time. I'd like to have the option to have remaining time/full length, like I had on MM4.
12- This also happens in MM4, and it really is a small issue, but I use auto-DJ and the playlist always takes a few seconds to resolve, but while it's resolving, many times the album art in preview won't show. I assume this has to do with operations order, but it would be nice if it would load the album art/track details before attempting to add the next file.
So far, the only issue that would stop me from using MM5 is the main panel issue. If I could get rid of the preview and place the playing list there, it would be all good. I know I did this last I checked MM5 (about 2 years or so ago), so maybe it's a temp thing/bug?
I actually like MM5 in general. It needs some customization so I find it aesthetically more pleasing, but the options are there, I just have to fiddle with them. And it has some nice addtions which I find superior to MM4.
That being said, on to the main reasons I haven't switched to MM5 yet - adding new files to the library. For this I'll be comparing my experience with MM4 vs MM5:
13- First thing I do is use the "My Computer" node to travel to the temp folder where I keep my music. I then select all files and clear out the fields I don't want. This is done easily on MM4, as it has the "All" sub-node. I have no way of doing this on MM5. I can't select all tracks on subfolders.
14- After that, I use the "Case And Leading Zero Fixer 1.4.2" addon. This capitalizes all titles/albums and changes stuff like disc# or track# from "1" to "01". I have tried the case checker you provided and it doesn't capitalize all words, instead trying to change "the", "a", etc to small case, even if it is capitalized already, nor does it fix leading zeros. I can see the add-on on "Manage Addons" but I can't configure it. And I don't think there is an MM5 version of the MM4 addon either.
The next step is using the "Clear Field" addon to remove the lyrics, because MM4 doesn't have a way to clear them in a batch. MM5 does, so that's an improvement.
15- Next is comparing tags with discogs, using "Discogs Tagger" addon. There is nothing close to it on MM5. The autotag uses musicbrainz, which is much less accurate. There are lots of options missing, compared to the addon, like selecting which tags you actually want to fix. If I don't want the tagger to change the artist tag but just change the album/date, I can, but not so with MM5 tagger. In fact, the MM5 tagger is so underwhelming compared to the addon that it really isn't an option. Can't even select which release you want to use. crap_inhuman already said he won't be porting to MM5, so I don't think this point will ever satisfy me enough to make me jump to MM5.
Anyway, once tagging is complete, I once again select all files and then analyse their volume. This once again falls under the #12 issue.
At this point, I move the fixed folders into the library, where MM will auto-organize them. I haven't tried it yet on MM5 but I have no reason to expect any issues there.
Then I check a few playlists I created just for library integrity checking (like tracks using ´ instead of '), fill in a few tags like genre based on my library.
16- Last thing I do is use the "Tagging inconsistencies" addon. This looks up for a lot of potencial library issues, mostly related to mistagging files. Stuff like files having the same disc and track# (usually because I forgot to change the disc# of the second disc), multiple album art or art not marked as Cover, inconsistencies with genres (like having "Indie; Rock" tracks on library and then adding one with "Rock; Indie"), inconsistencies with names (like having an artist "MONO" and adding an artist "Mono", etc. This is hugely helpful and I see nothing of the sort in MM5, nor does it seem like this will be ported to MM5 either.
I then usually clean up the library, which MM5 should also do without issues.
Small improvements I did find with MM5:
a- Adding artist that have a "/" in the name actually works. In MM4, even though it's configured to only use ";" as multi artist separator, if I add a band like "De/Vision" it will always add it as multi artist. I always have to change artist tags again after adding to the library. MM5 added the tags correctly with no issues.
b- When I delete an album, it actually deletes the folder if it's then empty. Not a big issue, but a nice touch.
All in all, assuming I could fix the whole playing thing with the main panel, I like MM5. I would switch to it were it not for the huge problems with tagging. The main reason I've used MM all these years was because MM made managing my huge (300k songs) library easy. And the addons used definitely speeded up the process of cleaning/adding new stuff. But I don't see an alternative in MM5.
To use MM5 I would have to switch to MM4 to clean/add tracks and then switch back to MM5 for normal usage, which is too much of an hassle, honestly. Not to mention that, occasionally, I spot an error on a playing file and have to fix it, oftenly having to use Discogs Tagger for it.
Anyway, I would like to know your opinion on my... well, opinions

And if there is anything being considered as an alternative, expected time for said alternative or if that alternative already exists.
As I said, I do like MM5 and would like to upgrade. It's just these issues (mostly just the tagging ones) that are preventing me from doing so.