I'd suggest negotiating (aka threatening) before leaving. They may give you a deal if you've been with them a long time.
I've been with AT&T a long time: I have an iPhone 4S - 2GB data/mo, more phone minutes than I ever get close to using + 4G aircard for my laptops - 5GB/mo data. $99/mo. I'm happy with their service; Verizon was the worst in customer service.
"I jump in it!" Russian Billionaire, DirecTV Commercial...