Just to add more GV experiences to the thread...
I got the invite last week, and activated it on my account. I already had a Google Voice number, which I've been giving out in place of my primary numbers, so I activated that number without porting my Sprint number. I like having the extra layer of privacy between the callers and my real phone numbers. If I want to block someone from calling me, I can just do so at the GV level, without affecting anyone else.
Plus, being in beta, I wasn't ready to hand my Sprint number over to Google...yet.
I had been using busy/no answer forwarding from my phone to my GV voice mail. When I activated GV through Sprint, I was instructed to deactivate the forwarding. Sprint is now handling it directly. Also, my GV number shows up on caller ID displays when I call someone else.
Like others here, Google Voice became the receiver of all of my SMS messages. The upside: I now get my messages to my Epic, my Galaxy Tab, and through the Web UI, so I can access them from almost anywhere. But like others, I would like to have the option to use Handcent on my Epic, unless the GV app is updated to allow the customization that Handcent offers (like the ability for different senders to have different notification sounds).
So how does MMS work?
I sent two messages from my Tab to my Epic's real phone number, as opposed to the Google Voice number. One was plain text, and one MMS. The plain text went to the Google Voice app, and the MMS was handled by Handcent.
I then sent MMS from the Tab to my Google Voice number. Nothing happened--no error to the sender, no alert to the receiver. It doesn't even get forwarded to the phone's regular messaging app. This needs to be fixed.
It's a good start, and definitely a much simpler process for those of us who have been using Google Voice already. Get some of the messaging bugs worked out, and it's worth getting/keeping.