T-Mobile is the 4th Largest cell phone company in the U.S. followed by AT&T - Formerly Cingular, Verizon Wireless, and Sprint-Nextel. It is currently a major supplier of Android phones (G1, Nexus, MyTouch, etc.). Their signal as of 2010 is not that bad, but lets be honest... What cell phone company actually gives you coverage everywhere you go? Yes AT&T may have coverage in some areas T-Mobile doesn't, but likewise T-Mobile has coverage in areas AT&T doesn't. Outside of metro areas and far into the rural areas, you do start noticing T-Mobile has a small footprint but today's plans allow you to use AT&T's network (known as roaming) for free. That's usually for calls anyways. They may charge roaming for Data but that may be in the plan details themselves. Usually though, most networks offer coverage in the same areas in this day and age.
T-Mobile is the one of the 2 major cell phone companies in the USA to use GSM technology, the other being AT&T. So it is very common for iPhones to be used on T-Mobile and G1s being used on AT&T, provided you did a SIM Unlock.
T-Mobile also offers wireless internet. They have hot spots (T-Mobile Hotspots) which are found in Starbucks, FedEx Kinkos, airports and other business locations. They also offer Internet service through a USB stick that you connect to your laptop. It is not the fastest nor the cheapest, but it is sold for the businessman who cannot access a hot spot.
Verizon customer: Haha, they obviously have low T. I have got 5 bars!