Voice: about 20 minutes a month on average.
Data: Assloads. But I am not happy with the expensive voice plus expensive data. So I killed all data on my phone and just pay the voice now. Contract is up in Oct. If they want me back, they better find a way to get unlimited data for cheap and very limited voice for dirt cheap or free.
Otherwise I get a Tracfone and carry my netbook around in a backpack. Every place I go has Wifi.
I had the G1 briefy and while I loved the phone it was the same problem. I hate paying for voice I'll never use, and unlimited data is too much, especially when I do a lot on wifi anyway. Including make UMA calls, so thats another reason I dont need many minutes each month.