sure... but it's slower... there's articles out there in each carrier's performance tests and I think you'd be really surprised if you were to at least look into it.
T-Mobile rocks in my area, in fact T-Mobile's 3G speeds are faster than ATT's and you factor in their 4G (Not even LTE) it screams.
I find it strange how people WANT to pay more... It's like trying to get someone to at least try a COKE over PEPSI. People like their brands.
people don't want to pay more, they just want to pay more for better service. for me, T-Mobile only has EDGE and very slow HSPA (EDGE-like speeds) where I live and for most of my commute to/from work (I take the bus everyday). Whereas, AT&T has fast "4G" (6-10Mbps down) and then LTE when I get to downtown Denver. Sure, T-Mobile has fast "4G" in downtown Denver, but it's not worth having EDGE everywhere else around the metro or where I travel.
But, I know this will be different for different places. I'll definitely keep an eye on their network here and will see what it looks like when they upgrade to LTE and maybe get those pesky EDGE-only towers to 3G/4G.