Having tried a ton of car apps, I can say that Vlingo is the best. My 2 cents:
Wake Command while you drive
Seems to do a better job recognizing my voice then google actions
Can't add custom buttons to home screen
Can't tell it to open applications like Win Mo
Does not give you an option of which text message program to use (This is my question!)
The one thing that holds me back is I use google voice for all of my SMS. Problem is that Vlingo doesn't ask me and uses the phones native message client. How can I change this or is there an app to divert text messages to voice versus the native message client. It does ask me if I want to use handscent but not google voice.