Depends entirely on your location, so for the most part, aside from implemented speed limitations (eg. T-Mobile's 4Mbit/s download cap), the replies you'll receive will not necessarily reflect what you experience.
Best bet is to try, or ask friends who use the network in the same area. As for online coverage checkers, they're only estimates, and may over/under-estimate the service you'll receive.
Freedom's suggestion of getting some PAYG SIMs is your best bet.