The two above posts nailed it on the head!
The Nexus S is a great phone. Yes, the lack of a MicroSD slot is kind of a drag, but most people wouldn't even use it. (Most normal users)
Like the above said, the fact that you get updates right from google is worth the phone alone. The Nexus series will usually out perform any other android phone because that, and because android is coded for the hardware it uses.
I feel like samsung has been slow to the 2.2 update for its glaxay series because 2.2 was coded for snapdragon processors, not hummingbird, which is why when 2.2 first leaked for the captivate and other galaxy s phones, it was really slow.
if it wasnt for AT&T migrating frequencies in my area, id have the Nexus S right now, no questions asked.
The cell phone game is an interesting one. If you love phones, and phones alone, Tmobile is probably the best because they usually get the best selection of android phones (most variety) and you can use the majority of unlocked phones on their 3g. That being said, in some cases Tmobile sucks for coverage, and if thats what you are after then you need to look else where.
personally, if i didnt work for AT&T, I'd probably be in Tmobile right now. Maybe verizon in a year or so if their sim cards can be put into other phones and work. Their fast network is enticing, but the fact that they get the least selection of phones (least, but a good selection) kind of turns me off