T-Mo in HTX is great. good signal, good speeds.
north and east of the city (east of US59 and north of HWY90) has some coverage holes, but that's the overall network. some heavily-treed areas like the Woodlands and Kingwood will have intermittent signal loss, but again, so will a standard wireless phone. when i have a signal (almost always), it's never below 2 bars, and rarely that low.
Cingular's coverage is vast as well, but i have not used an orange 'berry.
as with any service, pink and orange occasionally swap as to who's "bangin' more bars", but areas of zero signal are few and far between. considering pricing, i'd say pink is the new 'berry.
you can always go to t-mobile.com, and plug in some addy's to the
Personal Coverage Check. then go to cingular.com and do the same. oh wait... scratch that last part.