[Verizon Samsung Galaxy S III] ac21365:
I recently jumped from Verizon to T-Mobile and pretty much all comments thus far are justified.

They've added -some- sites in recent times, but the "re-farm" you seem to have heard about is primarily a massive equipment refresh for about 85% of the existing WCDMA sites. Unlike Sprint's Network Vision or AT&T's "add an LTE panel to an 8 year old WCDMA site" rollout, T-Mobile is doing a -complete- equipment refresh on said sites using either Nokia Siemens Networks Flexi Base Station solutions or Ericsson's Antenna Integrated Radio setups. For the sites that get the new equipment, coverage could see a 10-15% improvement in coverage and in-building penetration due to each of the NSN/Ericsson setups having the radio at tower-top.

T-Mobile will do you well if you are a city-rat. The lack of rural expansion pre-AT&T failed merger was due to the cost of infrastructure and backhaul when the only spectrum you have is high-frequency (PCS/AWS). Even in areas of excellent coverage, there's still a few limitations: Deep inside buildings, attenuation -will- eat the signal. Terrain is another big one that affects all carriers... if you happen to be in a place where terrain blocks your line of sight, you're not gonna get a good signal. Now, the equipment refresh is to facilitate LTE, as both setups are capable of LTE-A out of the box. T-Mobile had to sit on their hands for 18mo and not do any proactive system/coverage upgrades due to the AT&T merger debacle, but it was a good thing as now that they have the money and spectrum from both the merger fallout and MetroPCS plus new equipment, they're good to go. So, depending on where you live, work, and play, T-Mobile could potentially be great...

but...

Unfortunately, when all you have is high-frequency spectrum to use and a network that hasn't seen significant expansion since 2009, this is going to happen. As with all carriers, one man's perfect signal might have a crap signal from another carrier in the exact same spot. Happened to me: with Verizon, could pull 30mbit LTE from bed, but at work I could kill an entire charge with light use because the phone had crap signal on all 3 services (1x EVDO and LTE(whose average speedtest at the desk was 2-3mbit down)). Meanwhile, terrain eats T-Mobile's WCDMA signal at my house(edge only), but I pull 3x the speeds on T-Mobile because of a near perfect signal than I could with Verizon at work.

If you have a friend with T-Mobile, treat'em to lunch and ask them to take a ride around town with ya to see how the service is where you go. Another option is to borrow someone's unused, unlocked T-Mobile phone and grab a sim, sign up for the $30 100min/unlim data&text online and carry that around with you for a bit.

For someone like me who -rarely- leaves metro areas when I travel, T-Mobile is great. For you, it could be different, just take the "ZOMG SPOTTY COVERAGE TMO SUCKS" with a grain of salt, and understand that no carrier is perfect everywhere, even if those comments for that individual are justified.

Forgot to add: T-Mobile won't see any WCDMA expansion for the foreseeable future since their current focus is on the equipment refresh on existing sites. The next round of expansion will come after the 600MHz auction next year.

Shoot me an IM/PM if you have any more questions.