I did it. Initially it was so I could get out of my AT&T contract and get back in later on a cheaper plan, but I just never bothered to leave T-Mobile because their coverage was decent enough and the prices were even cheaper.
There's some loopholes though that you have to be aware of:
1. You have to trade in a phone. And you aren't going to get full value back on your brand new iPhone if trade that in. If you or a friend has an old, beat up Verizon flip phone lying around, your best bet is to turn that in for trade instead. You won't get any money off of it, but you'll at least be able to use your existing Verizon iPhone (which is unlocked) on T-Mobile and not have to buy an iPhone twice.
2. You HAVE to buy a phone. Again, you could trade in your iPhone and buy a new one from them, but you will lose a lot of money that way. Or, you could trade in a crummy phone and buy their cheapest, crummiest flip phone they have in the store, which is usually some really cheap LG model. LG is probably making tons of money off phones that never end up getting used for anything.
3. You have to submit your final bill that shows the ETF amount. Usually the first page, and the page showing the ETF charge is enough.
4. Takes up to 8 weeks to get reimbursed the ETF as a prepaid card. During that time, you need to maintain service with T-mobile and be current on your bill. You'll probably also need to pay the ETF before you get your rebate card.
If you can front the cash in paying your ETF for a while, it's a good deal.
Also, if you sign up, you can get someone to refer you (like myself) and you'll get another $25, AND free unlimited data for up to a year (bringing you bill down by another $30 per month).