I am with T-Mobile for about a year now, and the overall experience is ok. I pay less than with AT&T (including company discount). International data roaming is awesome, and with WhatsApp I can even get descent calls done with only 2G indicated data speed.
Even with GoGo you can get acceptable WhatsApp phone calls (on your commute of course), and for those that have family members (or girl friends) outside the US, their international plan is also worth a look.