I used to be on Sprint but moved to T-Mobile about a year ago. Currently I like it more than any other network for a few reasons:
1) I get excellent coverage and always see an H in my taskbar and minimally see an E. I do live in a big urban city so that has something to do with it, but I seem to get reception in places that my boss with an Epic can't seem to get bars at all. Check the coverage map in your area.
2) GSM means you can use more phones and you don't have to flash things or go through a crazy process to update the PRL and blah blah just to use an ATT or international phone (although you do lose 3G).
3) The only network you can use a TMOUS HD2 (with 3G), which is currently my favorite phone ever.
4) Pricing. I pay $60 a month on a no-contract price for unlimited data, text, night/weekend minutes, and enough daytime mins to be happy. Plus my girl is on tmobile too (nexus one) so our calls to each other are always free. VZW charges like twice that much with capped data. I can tether all I want with no extra charge too.
The Nexus S looks like an amazing device, and it is the ONLY phone I would replace my HD2 with. NFC technology is awesome. 16gb inand might seem stupid because no sd card expansion, but you don't have to mess with a2sd and ext partitions on an sd card or any of that crap, everything on NAND seems like the best way to go for me, although since it's a samsung they may not be able to port sense (so far all tries on galaxy S have failed), if you're into sense. Currently I'm running a fully stable Gingerbread NAND rom on my HD2 and I love (CM7) and I love it much more than any sense build I've used. That screen-off animation itself is reason enough for me
I'd recommend the switch, just make sure to check your coverage on the map on the site to ensure you're not in some backwoods no-reception area.