I've had basically no problems. As far as stability, I'd say I have few to no crashes with it (maybe every once in a while, but not really any better or worse than 1.5.0.x was). The only real problem I've had was getting a couple of random extensions I use to work, and even that was pretty minor trouble. All of those that didn't work initially, I could force to work (and they caused no problems) by using the
Nightly Tester Tools extension (which among other things lets you override the "test" to see if an extension is compatible and try it yourself). I don't use too many "hard core" extensions... Just Adblock, Filterset G Updater, Greasemonkey, and Flashblock (all of which did have compatible versions available on their web sites, though the auto-updater didn't always find them). I also use the Manage Folders extension and a handful of other little things that haven't been updated, but worked fine after I forced them using the Nightly Tester Tools.
Most of the changes in the new version are subtle. Nothing super-wows you, but I am quite pleased. I really like the new spellchecker for filling out forms (helps writing posts like this one). I also like the new RSS support (and the Live Bookmarks finally show formatted text rather than all the &apos tags). The new "look" is nice, but nothing to jump around about. The new tab stuff is nice too (they no longer get infinitely small and it can remember open tabs through relaunches and in the history).
My only "complaint" is that until they release it, it installs as "Bon Echo" instead of Firefox and lives in a separate folder. That's just how they did "Deer Park" before 1.5 came out. It's fine, but if you want to keep the "stable" firefox installed, you'll need to make a separate profile for either Bon Echo or for Firefox-stable (which is what I did), because the profiles are not compatible.
You can read some instructions on doing this on the build forum here:
http://forums.mozillazine.org/viewtopic.php?t=462431&sid=0af502cdd9c3d7f66f3401da5a6804c1