I was on AT&T and had a week or so left in my contract when I got my N900. I found it worked pretty well on edge, though web browsing was a bit slow. Even one day before the end of my contract, AT&T would have charged me hundreds of dollars to get out of it--no prorating. So I waited and used edge.

When the contract ended, I switched to T-Mobile and got the no-contract plan. All in all, cheaper than getting a new phone and contract with AT&T. Here in Austin T-Mobile is almost as good as AT&T was--a little worse on the coverage and I am occasionally dropping calls or failing to connect, neither of which happened on AT&T. My non-iPhone AT&T 3g coverage was actually very good for all of 2009.

In any case, you won't notice the difference on non-multimedia stuff. Stick to the mobile sites for big websites (e.g. touch.facebook.com or m.yahoo.com, etc.) and the performance is decent. And using apps such as facebrick and conversations for chat should be perfectly fine (facebrick wasn't out when I used edge, but it should be ok).