I'm making the jump from Verizon to T-Mobile today. 
On Verizon I have 1 unlimited data line, 1 2GB line, two dummy basic lines that we don’t use (opened them up to activate their phones on our regular lines), unlimited text, and 700 minutes per month.  We pay $200 per month.  On T-Mobile I’m getting 1 unlimited high speed data line, 1 2.5GB line, unlimited text, unlimited minutes.  With my unlimited data on the Jump! program, I’m saving about $50 per month.  Plus I don’t have to deal with a stupid borderline-thief company locking down every bootloader in sight, not to mention refusing the Nexus line and throwing us a phone about 6 months after everyone else.  Yes, they have the most extensive network, but nowhere near the fastest.  I tested out my friend’s T-Mobile phone in my house and place of work, and on speedtest he was getting about 17mps faster than my crappy Verizon phone.  It’s like a breath of fresh air, leaving that disgusting company.