@cmacv,
Handcent can be a bit laggy and has many unused custom features. Main thing i like is after when you send a message via sms popup the screen turns back off, unlike Handcent which stays on, and you have to return to the home screen then manually turn it off. And for quick replays its a big difference.
If you want it to look like the iphone sms service hancent does that, but i found it gets annoying after a while, and turn back to stock