Well, if you unlock the phone, you can use a card from T-mobile. I don't think you get the same speed as AT&T unless you root and flash a T-mobile modem or something like that. Check the XDA forums for more info on what is required.
I got my AT&T Note used, I think I used cellunlocker.net or some similar link to unlock, using a T-mobile SIM I bought on Amazon.
I don't really use my Note as a phone, for me it is a portable wifi tablet. But I do keep a prepaid, $2/day plan going, (cost $40/year) so occasionally I can access data locally when I'm out and don't have a wifi connection available. I only usually do that if there's an eBay auction I need to bid on. So I know a T-mobile SIM *does* work.
I'm sure you could get other, more expensive plans though.
Sent from my Samsung Galaxy Note (Thing Two).