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.
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.