I just saw a link to this Series 90 theme for my 6600 on the MobiIRC channel and thought it was a great thing to try. I just loaded it up and it doesn't look that bad, actually, I'm going to keep it for a while.
This is the first third party theme that I've tried out. People are keeping them pretty close to their vest. I'd love to hire a dozen out-of-work graphic designers to crank them out, but I'm not sure there's enough 6600s out there to actually be a viable business. Besides, Nokia's 6600 Theme Studio is a disaster to use. It's in Java and even on my 2Ghz Celeron with 512MB of RAM it runs slow as death and is pretty much unusable. I haven't used Sony Ericsson's theme creator for their T610/616 and Z600 models, but I bet you it can't suck nearly as much as Nokia's. Oh well, I got what I paid for, right? ;-)
Anyways, nice theme, It's definitely fun to be able to skin the UI like this a bit. Not too much, but just enough to give it a jazz.