I got a Google Voice Invite over the weekend, so I setup my google number and whatnot, it was pretty easy, I did have some problems, I tried calling my mom through the interface, what was supposed to happen is it was supposed to call my cell phone (I had selected), then from there it was supposed to connect me to a call ringing to my mom.
Unfortunately, I ended up with a voicemail of a phone ringing, and my mom didn't recognize the number so she didn't answer....
It did have my google contacts which is nice, I tried SMS'ing my cell from their interface and it appeared to work fine.
I'm not sure how I feel about calling into a phone number then having that phone dial out to another number, if there was a way to make it more seamless as far as the outgoing calls I might like it more.
For now I'm going to stick to YouMail for my voicemail provider (at the moment). The nice thing about YouMail is that I don't have to change my number.
Once Google integrates things better I could see myself using this more, I know they have an app out there for the G1 that will use your default contacts list, and then from there you can just make calls directly, but I am currently without a google phone, so unless they find a way to make this app work more seamlessly with more phones it's in the "Its cool and I might poke at it a bit but I probably won't use it steadily until it's got more usability"
Let me know if you got an invite and what you think of Google Voice!
Once you've got your one phone number for "life" might be a good idea to get some Google Business Cards