So far I've only heard complaints, I think that if it ends up anything like it's big brother on the PC we'll be fine. I'm on the EVO running 2.2 and Firefox runs fine for me, it does however lag a bit especially when scrolling the pages to fast (I get the checker board effect, or a grayed out section). I don't hate the UI, but it could use a few improvements. I for one do not like the side scrolling for bookmarks or tabs let alone to go "back or forward." I like how xScope has accomplished this buy adding a bar at the bottom and tabs at the top it you chose to keep them.
I feel the Mozzila team needs to get in touch with their beta testers (us) and read a bit on these forums for some suggestions. I'd like to hear what the possible future may hold for Firefox 4 on Android, wouldn't you?