Following Steven's latest addition (=user-configurable buffer size) I'm seriously considering giving v3.0 a try, so some obvious questions arise:
1. Can I install it along-side the v2.5 version?
You can have both open at the same time, but I would suggest not making any file changes (tagging, moving, etc) while both are open. You can open both while moving around v3's design elements so that you can match what you have in v2. You can have both open for comparison purposes. If you're going to tag edit, do it in v3 without v2 open. Then close 3 and open 2 if you want to check the changes.
2. Will they use different DB/Settings? Any way to migrate the v2.5 settings/DB to v3.0?
There's a MusicBee3Settings.ini file that is separate from MB 2 settings. You can change whatever settings you want in v3 and they will not cause any 2.5 settings to change. And visa-versa.
3. How stable is it?
I'd have to say it's pretty stable. I stayed away from working with some of my "irreplaceable" files on v3 for a while. And then when I did, I made sure I had a backup of the file before touching it. Now I don't do that. As a matter of fact, I'm now using v3 almost exclusively. About the only time I've opened 2.5 in the past couple of weeks, was to compare skin changes I was making. Or to compare the behavior of one versus the other.
4. Any missing features?
I think at this point it's safe to say that if you could do it in v2.5, you can do it in 3.
But remember, Steven has yet to call this version a beta. I would call it a late alpha.