My reasons are simple:
1) BIG keyboard that slides out and tilts
2) Removable Battery
3) Micro SD slot
4) Sprint vs ATT (lower bill every month!! and better service!)
5) It was free!!!
6) You don't need an additional purchase to do Excel, Word or Powerpoint
7) Slingbox works WAY better on the TP2 than Iphone (probably due to AT&T)
Easier integration into a Microsoft Networking environment (Outlook conduits, Exchange servers, etc)
9) You can tweak the OS with custom roms
10) WAY better battery life!! (my coworkers have Iphones and they can't pass a power outlet!)