Switched last year from AT&T, once contract ran out.
T-mobile is most cost effective on a large family plan. We pay our own lines but share the plan with extended family and everyone is trusted to pay their share online. The marginal cost per line (with kick-back, aka <2GB data usage) is just $10. For, say, 8 lines, the per-line cost (with autopay and kickback) is like $25?
I already had netflix so the free netflix benefit is money in my pocket
The t-mobile tuesdays are often stupid ($2 dunkin donuts gift card) but sometimes useful
I travel abroad at least a few times per year, so I do enjoy the free 2G. It can be spotty, depending on the location, and I may have to fiddle with data options/airplane mode, and it's obviously slow. But it's a godsend when you just need to look something up on the map, or google, and don't want to hunt down WiFi. It's pretty good for imessage/WhatsApp, and even if you have to send a normal SMS it's free.
Most of the time I'm on wifi, though, and the wifi-calling works really well for handling US business (i.e., calling US numbers)
Compared to my last trip to Europe on AT&T, when I got a local SIM card in Italy, it's much preferable to just arrive and have data, and obviously cheaper (but that depends again on the country). Wifi itself may be ubiquitous these days, but open networks that have actual internet connections, and don't have extremely slow/buggy captive portals, are actually pretty rare in my travels. Half the places that say they have wifi either don't have a network visible, it's down, or the internet is out, or they don't want to give you the password or whatever. Even airports are hit or miss because they are typically under heavy load.
Overall, happy with it. If you do switch, consider waiting for one of their 2 for 1 phone or other promotions, which usually require adding a line. Also, go to an actual store and ask them to waive the SIM card fee or at least something non-scandalous like $1. Online they charge you ridiculous amounts for the SIM cards.
If you buy a phone from them, and paid off, unlock is easy with online chat or call.