Having both, I can attest to TMobile coverage being SIGNIFICANTLY spottier than Cingular (which is now called AT&T, a case of the big fish spawning a small fish gobbling up bigger fish, gobbling up still bigger fish, gobbling up the big fish).

AT&T comes close to nationwide coverage (but still not as widespread as CDMA which, alas, is not GSM); the coverage map can be found here: http://www.cingular.com/coverageviewer/

AT&T has more than quadruple the number of cell towers. T-Mobile's network primarily emanated from what used to be Voicestream, which had under-invested in its network.

The proof is in these maps. While coverage in major cities tends to be ubiquitous in both cases, the difference quickly becomes apparent. Take a look at the most densely populated state in the country (New Jersey). Zoom in a few levels. You'll see T-Mobile's coverage looks like a kid with chicken pox. A bunch of green spots but these spots are not continuous. There is "dead zone" (or "one-bar if you're lucky" area) in-between most of the spots. Most curiously, when you travel along major highways, turnpikes and Interstates, the situation is the same. Simply put, if you drive, you should expect to have areas with no service, and you should expect dropped calls.

Look at AT&T's coverage. It looks like someone spilled many cans of orange paint ... also you'll note significant areas of 3G coverage. And look along all the major Interstates and Turnpikes ... they tend to be in completely dark-orange ("best coverage") state.

Don't believe me ... believe the maps. And when a densely-populated state like NJ has AT&T being way way better than T-Mobile, just imagine when you move into the bigger and less-densely populated states!

Also, make sure your phone is a tri-mode or better, a quad-mode phone. While most of the world uses 900 and 1800, US uses 850 and 1900. Tmobile until recently used only 1900, but has been rolling out 850 as well. If you have a standard European handset, you're SOL. 1900 (like 1800) is a frequency that is dense and good for cities; 850 (like 900) generally covers larger, rural areas, but with lower density. T-Mobile's lack of 850 coverage is a prime reason for spotty coverage outside of cities.

----
Another FTer told me I am incorrect about AT&T being the new name of Cingular:

I stand by my assertion.

Cingular did indeed buy AT&T (Wireless) ... but now AT&T (the long distance business) has bought Cingular and is renaming it itself AT&T! Look on the top of www.cingular.com ... "Cingular is the now the new AT&T" ...

Cingular did buy AT&T wireless. That's why your AT&T brand name was going away, a process that had been taking 3 years, and has now changed course 180 degrees.

However, Cingular was owned by BellSouth, which got bought by SBC, which also bought AT&T, and renamed itself AT&T. Confusing? Sure.

SBC Communications (formerly Southwestern Bell), bought the long distance part of AT&T, and renamed itself AT&T. SBC (AT&T) then bought Bell South, which owned Cingular, which owned the former AT&T mobile business.

You get the picture?

So now that SWB (calling itself AT&T, which it owns, including the brand name) owns BellSouth which owns Cingular which owns AT&T Mobile (but not the brand name), they have decided to reverse course and drop the Cingular brand name and call everything AT&T. Here's more detail:
http://www.cbsnews.com/stories/2007/...n2356736.shtml

You should see the Cingular brand name disappearing over the next 6 months ...

If you want a more humorous view of this, Steven Colbert did a great job explaining the history:
http://www.youtube.com/watch?v=z5ZhxewK2aE

Or read Alice in Wonderland about putting Humpty Dumpty back together again :-)