The 3rd party GV Voice app that I have (and was pulled) does full SMS, allows changing of what phone numbers ring, lets me dial anyone so it shows up as my Google # to the recipient, and allows visual voice mail access. If I use it to call a number, Google calls my phone first, then connects the call.

I would imagine Google probably had some additional features, possibly including push support so Google Voice SMS would be just like AT&T's $1600 per MB SMS service.