After two years of daily use, Handcent SMS has a serious competitor.
Go SMS Pro
The apps is free and you don't need to add anything to it. But if you want to, you can make your own themes for free or buy themes for more appearance options. Plugins add extra function. I'm still checking those out.
Performance is good. It's quick. Feels light. I haven't tried all the MMS functions yet. But so far, so good.
Here's a screen shot of my phone with the (paid) "Future" theme.