I have a Nexus One and its a great phone that you will enjoy
The flexibility it gives you to buy it unlocked and save money with the $50/mo voice + data (w/ free txt through GV) no contract plan is what is keeping me with T-Mobile so far instead of jumping ship.
I hate the idea of spending $720 more, contract wise, over a 2 year period (even if the N1 is $320 more upfront) I'd much rather just spend more upfront to save $400 over the 2yr period.