The performance improvements have been great, but I have to admit the UI changes are highly concerning to me. Fortunately the FF extension APIs are the best around, so now I just can't use Firefox without "Classic Theme Restorer".
I personally never minded Australis, since I've been used to it via Thunderbird (which rolled it out first). That said, it really doesn't affect me that much since I use Tree View Tabs anyway (and I don't use the menu enough to really care beyond having nice big buttons to click on when my morning coffee hasn't kicked in yet).
The performance improvements have been great, but I have to admit the UI changes are highly concerning to me. Fortunately the FF extension APIs are the best around, so now I just can't use Firefox without "Classic Theme Restorer".