Hi moester, there is a thread or two on this topic somewhere, but IMHO if nothing is holding you back definitely jump ship.

I have a family plan on ATT which I used my N900 on for about 2 weeks, I then purchased a contract free Even More plus line from Tmobile last week to test 3G and the difference is night and day. T-mobile 3G is lightning fast here in NYC compared to ATT (high percentage of iPhone users here methinks). The ability to use data and voice simultaneously is another benefit. You really get to use all the power of the N900 to do things like stream internet radio and video, as well as VOIP services over 3G which is simply not up to par with the 2.5G of EDGE on ATT.

I would totally jump ship if I could but I have to convince my wife and her brother to switch and they both have unlocked s60 phones on ATT bands