Didn't have any problems with 0.9...
So far, no probs with 0.9... I imported my old bookmarks and installed my favorite extensions (Tabbrowser Extensions, Diggler) without any problem. I'm no expert, just reporting my experience using Firefox as my default browser. If there are any bugs, I haven't noticed yet.
Of course, I removed all traces of 0.8 first. Originally, when I installed 0.8 without cleaning up after 0.7, I experienced a few glitches, which disappeared once I did a clean install.
0.9's default interface is certainly spartan-looking (OK, OK, ugly)... the 0.9-compatible themes are available here. You can get there also using the link in the Theme Manager (in the Tools menu). Still too few, but I guess it is just a matter of time before the more are listed.
I feel silly saying this since it sounds obvious, but its wise to always restart Firefox after installing a new theme and/or extension. I've noticed that now when I download a Theme, and sometimes with the extensions as well, I no longer get the message prompting me to restart Firefox... :confused: