Thankfully T-Mobile is a carrier with some decent signal in my area, even if the signal at my home has degraded a bit in the past year. I currently have an EVO on Sprint - which performs nearly flawlessly by the way - so adding a FREE Nexus S courtesy of the wonderful people at Android Central might not seem like a big deal, but believe me it is filled with awesomesauce to be sure. (Is that sorta Ewwww?)
I left T-Mobile for Sprint because of the poor signal performance and multiple outages. I still believe I would get a respectable signal while traveling and having access to a GSM device wouldn't hurt in areas where my CDMA phone doesn't work.
For me... winning the new Nexus S with Gingerbread will give me my first opportunity to experience the Android OS in all of its natural glory, and being among the first for Google updates doesn't hurt either, even if the device lacks a few features like a removable microSD card (scratches head) or Bluetooth 3.0.
;-)