I just got my invitation to GV last week. It is a pretty amazing service. At first, I couldn't decide whether I wanted to just go with the VM version or the new-phone-number version. I decided to go with the full GV phone number version, since I realized that people can still call my direct lines without any adverse affects. Since I've signed up, not a single person has left me a voicemail.
The only one's I've gotten to play with thus far were the test ones that I sent to myself.
I haven't tried making an outgoing call from my Droid using the GV path yet, so don't know about the impact on voice lag.
Even though I haven't put my new Google Voice number to practical use yet, the time will come, eventually, when it will make sense to do so. In the meantime, I want to get an account for my wife, too. That way, we'll both be ready to go.
-SR-