I carried 2 smartphones with me during my trip. 1 on AT&T, 1 on T-Mobile.
AT&T:
-decent coverage, you'll get at least few bars. Obviously no signals once you're inside rides, etc.
-USELESS data connectivity in the parks. I was going to use some apps on my visit, and those apps depend on data connectivity. It's useless. Even at 3-4 bars on 3G, I usually get time outs/no data. The connection is flacky too, reverting to EDGE/disconnect completely very frequently. Even on EDGE, it's useless. The only times it's useful is when I got back to my hotel.
T-Mobile:
-less coverage than AT&T, although fairly minor. There are few spots where my AT&T phone can still get a signal while my T-Mobile phone showed no signal at all.
-Better data connectivity. Throughout my trip, my T-Mobile phone gives much better data connectivity, even during the peak hours in the parks. Sure, there are times when things slow down, but most of the time I can still get data connectivity and use my apps while my AT&T phone is timing out for the whole day.