Eventually, T-Mobile got whatever was needed with the local towers done. It STILL fluctuates wildly, but it USUALLY stays at a speed that makes things usable. At good times it runs quite well. It's better than the DSL we had. and cheaper.
Unfortunately, I guess that makes the best answer is to keep calling T-Mobile when it sucks, go through umpteen "support" people, generally be a pain in the *ss, and have graphs like the above that show how inconsistent their service is.
With a little luck, after 4 - 5 months, hopefully they'll get things to the point where the service doesn't drop to zero, and will mostly work most of the time! If you're lucky, it'll work well enough you rarely get buffering on the 480p tv, and downloads will be reasonably fast.