I am using Chomp SMS. It is a much better UI than the stock messaging app. I have used Handcent in the past but agree that there are just way too many options built into that program for text messaging. Chomp is simple, streamlined and looks a lot like the iPhone SMS program. (Lets be honest here, texting is the one thing iOS got right)
There are several options out there, now if the could just manage my google voice number as well.
Stoopid question - I too got Handcent. Is there a way to turn off the native messaging app? Right now I have two apps notifying me of messages etc. I want there to be only one operating.
I thought it was ok too. I like the way handcent looks though. There's a emoticon set you can download for it that makes them look like the ones on here. Pretty cool.