Month to month plans are not nearly as good or cheap in the US as they are in Europe. I've been doing a lot of investigation about this and it's frustrating because I want to buy a new unlocked phone here in the US but it's almost not worth it here because you still have to buy some absurdly expensive plan.
In Germany I was able to buy a 20 euro a month data only plan for my iPhone and I was perfectly happy.
I ended up ordering the iPhone 5 on Verizon at the subsidized rate ($300) and stuck it on my dad's Verizon account for $30 a month (cheapest data plan only) and then when I go back to Germany later next year I'll just cancel it and try to get out of paying the early termination fee. Even if I have to pay the fee it's only $350 which, added with the initial price of the phone, is the same price as out-of-contract iPhone 5.
There are both cheaper plans available and you won't be locked in for 2 years. Both, combined, are worth more than the ~$300-$400 phone subsidy over two years that the telco provides.
That's changed, unfortunately. Their "Value Plans" now also require a two year contract. They have no contract plans, but they are pricier than the contracted Value Plans. I was pissed to learn of this change when I wanted to switch to a plan that would allow tethering (I have the old G1 plan, which is unlimited data, but won't allow addition of tethering, and they shut down my ability to tether with the native tethering app or third party apps). Since I paid full price for a Sensation 4G less than a year before, in order to avoid a contract, it's pretty darned frustrating. And, let's be honest, T-Mobile is not a great network.
How did they shut down third party apps? I'm technically not supposed to be able to tether on Sprint, but I can run PDANet or FoxFi to tether over USB or bluetooth anyway, on a non-rooted phone.
They monitor traffic, I assume. I have historically used both PDAnet and FoxFi among others...all get shut down within a few minutes, and I receive a text telling me to sign up for tethering service. I'll try it again after rooting the phone, to rule out simply detecting all the common tethering apps, but I'm guessing they're detecting NATted traffic or similar.
I'm considering picking up a Nexus 4, rather than bothering with rooting and re-imaging the Sensation. I don't really like the Sensation, though all the problems I have with it could be due to software. It's a custom build with Sense, which I really don't like.
Anyway, it's a new phenomenon. I was tethering on T-Mobile for years without trouble. About two months ago, they started shutting it down after a few minutes of use, and redirecting all traffic to their T-Mobile site upgrade page.
