I agree with you that it's usable, but on my machine (an i5 iMac/16 gb/Debian) it's slightly slower than Chromium. Not enough that I won't use it, but noticeablely slower.
I also agree with the upstream comments about UI clunkiness. There are a lot of things that irritate me about Firefox that Chromium just handles better.
Mostly little things like the "Recently Closed Tabs" menu is buried 6 levels deep in the "Library" menu. And the completion list that pops up when typing in the address bar always seems to miss pages I've visited but includes things that seem unrelated to what I'm typing in.
