I downloaded handcent just because of the messaging issues. I allowed "pop ups." Every time I get a message it pops up on my screen and all I have to do is push the icon and it speaks the text to me. No need to unlock or anything. I pull it out of my purse in time to see the pop up and I just choose whatever option I want from the pop up (Read text out loud, or text to speech). It's pretty convenient.
I don't know how the driving mode tts works, I didn't get a chance to try it without Handcent.