I would recommend T-Mobile. I'm using a Blackberry Curve 8520 using their unlimited data plan, 500minute unlimited text plan. $70.71/month to month/pay as you go plan.
The EDGE service is spotty, like Boost but they also have sufficient selection of 3G and Android phones.
I am in a unique situation. I am using a GSM phone with GPS mainly as a location sensor. So, I don't need any voice minutes, just unlimited data, for as low a cost as possible.
3G would be nice, but don't even really need 3G.
Other than spotty coverage, Boost fits the bill perfectly. They have unlimited data for 0.35c per day.
The EDGE service is spotty, like Boost but they also have sufficient selection of 3G and Android phones.