I have been using MusicBrainz Picard for a couple of weeks now in order to sort out my MP3 library, and it has been working great! I have only a small handful of recalcitrant tracks left that resist identification, on a medium-large (7500 track) MP3 library that hasn't seen any serious metadata love in over a decade.
Here are some of the things that I really love about Picard:
1. The browser integration
Being able to do my research in the browser, then click on a link to send a chosen release/recording for a track back into Picard is absolute genius! I abuse the High Holy Heck out of this feature!
2. The ease of editing metadata, renaming and moving files
I almost don't need another utility to edit tags or organize files, because Picard already lets me do so much!
3. Sort fields
The automatic population of sort fields is a godsend. In the past, my renaming process was so tedious because I wanted my files ordered by sort fields, but at the time, no software I found supported it.
4. Automatic Cover Art Lookup
Only my more recently-acquired tracks have cover art. Everything from my CD library was ripped in a more primitive era before embedded cover art was popular. Being able to refresh those tracks with proper cover art is truly awesome.
5. The scope of the MusicBrainz database
MusicBrainz was able to ID many of my more quirky tracks, and give me lots more info on tracks that I barely knew anything about. I came in to this project with several dozen tracks with artist names of "Zzz Unknown". As of this moment I am down to one.
I do have a couple of suggestions that would make Picard EVEN BETTER based on what I found during my work:
1. Keep the UI responsive when adding large folders
The first time I opened MusicBrainz Picard, the very first thing I did was to add my main Music folder, which contains over 5,000 files at its root, plus a couple thousand more among subdirectories. When I added the folder, the UI went unresponsive for a long time. Since it was my first impression with the application, I almost stopped using it on the spot, though I am VERY glad that I didn't. This application behavior might be driving other users away during the first 5 minutes of their first session with the software.
2. Save the Workspace
It takes time to arrange everything exactly how I want it. Sometimes I think a track belongs to one release, but a couple of hours later I find some accompanying tracks that change my mind. Until things are finalized, I don't want the tracks to leave the pane on the right of the workspace, which a problem for a multi-day project, since Picard forgets about the workspace when it is shut down. Every time I start Picard to continue work, I have to re-add my music folders, re-cluster, and have Picard look up all of the album info, which takes several minutes. It would be REALLY handy if Picard could save the workspace on shutdown and re-load it on startup.
3. Allow manual addition of cover art
With Picard doing so much, it's a real shame to have to use a separate program to manually add cover art where cover art is missing, especially when Picard already has the ability to automatically look up cover art. If Picard supported manual addition of cover art, my workflow pipeline would be much simplified and I wouldn't need another utility until much later in the process.
4. Warn when trying to re-tag .wav files
I had a couple of rogue .wav files in my library that were giving me deja vu, because I didn't realize right away that they were .wav files, and I swore I was re-tagging them multiple times. I was right. Picard just happily acknowledged my requests to save tags to these files without complaint, and then just seemingly threw the tags away, as tagging .wavs seems to be unsupported. These files needed to be converted to .mp3 anyway, so it would be nice if Picard could warn about them.
5. Retry album lookups
When reloading my library into Picard (see #2 above), there always seems to be a couple of album lookups that fail, probably due to timeouts and whatnot. It would be nice if these could be automatically retried a number of times before giving up, instead of manually having to hunt down and refresh them.
Overall the dev team has done a great job here. Hopefully my suggestions can help add to that even more.