this is how it breaks down...
ATT service is lousy because its oversubscribed in nyc. If you try to use your phone around 5-7pm, its hard to connect. The good thing about ATT is that they supposedly have excellent customer service.
Sprint is cheap-- cell phone for the masses, lots of minutes, service is so so, customer service is atrocious. I think they have a bigger local calling area than the others service providers.
Verizon is by far the best in NYC area but it is also the most expensive (at least the last time i checked, and their selection of phones was limited).
A couple of friends of mine have Voicestream and they are very happy with it. I was happy with my Voicestream phone as well and I had the option of getting some nice GSM phones to use with the service. They give you a lot of minutes as well.
Don't forget about Cingular-- they are starting to roll out in nyc. I have no clue how their service is or will be.