I have tried it. Didn't notice anything that chompsms doesn't have..
Well, for one, it pops up a small box with your incominng sms automatically, even if your phone is turned off. With ChompSMS, you can get this, but you'll need to download another program, SMSPopUp.
Even then, with SMSPopUp, if you want to reply to an incoming sms, you need to have it launch ChompSMS. Handcent includes a "quick reply", where you can respond from the pop up window if you don't plan to have a long conversation, without launching the full app!
HandCent allows you to customize notification sounds for any individual, as opposed to one sound for ALL incoming sms messages. Know who's texting you before you turn on your phone.
HandCent, when open, has a button to quickly call whoever you're sending messages to.
Handcent also posts pictures of the person you're sms'ing with, if you have their photo on your phone.
Includes several themes...
-Classic
--Handcent
--Iphone (this is personally my favorite)
--Android
Includes "Text to Speech"
Allows you to use your own background for use in the program.
Send MMS messages straight from Handcent. ChompSMS, when you indicate you want to send an MMS, sends you to the built in app.
anyways, I'm sure there are a few other differences, this gives you an idea of some things that differenciate the two programs.