I am using T-Mobile for nearly 3 years now, and I am not always that impressed. Yes, there is very good coverage both at home (base station just 200 meters away) and work, but where I use mobile web most - on the Tube, that is - isn't that perfect. However, I am also quite convinced it's also a matter of rather crappy radio hardware that Nokia E71 has (!), hence I am looking forward to see how things will get with N900. Anyway, I once created this: http://adl.pl/tmobile_coverage.phtml - and submitted to T-Mobile directly, however no feedback, whatsoever...

Speaking of merging Three and T-Mobile infrastructure - personally, I can't see that happening. I know a spot close to my house where both T-M and Three have two separate base stations (lamp-post masts), about 50 meters apart from each other. Once they announced merger (two years ago that was?), I thought that particular and obvious spot will get merged as one of the first, yet nothing has happened so far...

Btw, DO NOT ever believe in coverage maps on network websites! These never take many aspects under consideration, and nobody will actually be able to tell whether you'll get good/bad coverage until you check this yourself. Would you ever imagine that T-Mobile has barely any coverage within Nokia Flagship Store at Regent Street (deep inside, next to checkouts), in the middle of London??? I couldn't believe that either...