Like the customization (the bubble theme has grown on me), background image in sms, and most importantly the REMINDER FEATURE. I come from "dumb phone" world before the Droid so I'm used to reminders. Having great options for that is a huge plus for me.
Honestly the program has almost "too" many options, but I guess it's better than not enough (i.e. default sms app).
The text-to-speech is something I have only used a couple of times and it seems to work quite well for short messages.
I never tried chompSMS but I understand it's very similar to Handcent. They are free, try 'em both!