I have mine unlocked and I'm on T-Mobile. I used the original software unlock on the night that it was released.
It works fine, there are no issues or problems at all - except the voicemail light comes on (whether or not there's voicemail) every time I reset the phone (which has been twice so far - once when I got it and another time to enable YouTube). I just have to call voicemail, leave a message and then erase it and the light goes out. Other than that... no problems at all. Everything (except Visual Voicemail - which is an AT&T feature) works as it's supposed to. I'm very pleased with the phone.
If you have more specific questions, I'd be happy to answer them.
For what it's worth, I went to the local T-Mobile store the day that I unlocked it and asked about data plans. They quizzed me about my needs and kept asking what phone and I finally showed the salesman my iPhone with the T-Mobile logo in the corner and he thought it was really cool. Several of the guys working at the store looked at it and thought it was awesome. They made me feel almost embarrassed about the attention. They spent a while on data plans and finally recommended the $20/month plan with unlimited Edge (and Wifi access at T-Mobile Zones).
Several of my co-workers have unlocked theirs as well and are on T-Mobile. No one has any complaints that I know of. The unlocking process was (is?) a bit scary though. You have reset the phone a fair bit and I thought I' bricked mine (killed it) for a while which had me a bit nervous.