I was on T-Mo's $30/mo Unlimited text & data + 100 Voice minutes plan. If I used up the 100 voice minutes, and I regularly would by a phone call or two, additional minutes would cost $0.10/min so I kept 20 bucks in the account just for insurance. Yet... ever time I went over 100 minutes I was cut off from voice. In some cases data was cut off too. My buffer never kicked in. I would end up having to renew early or at least manually. Auto-renew did not work. This went on for nearly a year before I got sick of it.
So then I looked at my data usage (on the T-mo website) and it was always un 5 GB/month. Well under. So I decided to switch to the $30/month unlimited Voice + 5GB data plan. This should work... but it didn't. My data was cut off after two weeks. I had somehow exceeded my limit even though I NEVER had before. And guess what? Under this plan they don't let you see your stats. You can't even see how much data you've used when you log into your account.
So I gave up on the bargain basement plans and went for the 40/mo unlimited voice/5GB data w/throttle instead of cut off. I consoled myself by thinking that I'd at least have unlimited music streaming & international data but... NO. That only kicks in at the 50/mo level. Found that out the hard way.
So now I'm paying for $50/month unlimited talk/text/data (only 1GB at 4g) plan, mostly for the privilege of no monthly billing hassle. This is really only slightly cheaper than Sprint, last I looked. I am abusing the free music streaming though and I have two international trips planed where I intend to use data.
It's still not a bad deal but my take away is this.... T-mobile will still nickle & dime you to death as well as the others and their low end plans aren't worth it unless you are patient and diligent. I'm also pretty sure they are playing games with your data stats.