I've used Opera 10 Alpha a lot lately, and it is quite good. It feels the fastest of the "big 3" mac browsers, but it was crashing on me about once a day, under heavy usage. If the final version is more stable, I'd switch back.
Safari is decently fast, but it just lacks a lot of functionality and expandability. It doesn't even save my sessions if the browser crashes! (the other two do)
Firefox can be painfully slow on this 5 year old Powerbook, but it has been rock solid stable for me. And I've become quite attached to having so many awesome plugins (skipscreen, adblock, downthemall, etc).
I'd like the webkit rendering engine + firefox style plugins, now that would be something (maybe google chrome will do this?)
Looks like Opera is skinnable, spy:
http://my.opera.com/community/customize/skins/