There are several things I would consider before switching.
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4. T-mobile is a bad value if you decide to upgrade your phone every year. Here's why: the plan will only pay off if you keep the same phone for over 2 years, if you pay the full price subsidy for your phone every one year, it's a complete wash in terms of money saved by switching.
I really considered switching from Verizon to T-mobile to save about $50/mo. After being realistic with myself and actually doing the math, it was a total wash, based upon upgrading my phone every year. Even if you upgrade every two years, the year that you upgrade is a wash.
Also keep in mind your sanity. Is it worth saving the money if your getting dropped/missed calls/spotty service/poor data speeds?? T-mobile has a lot to prove still.