The story: Hafner's had great success at the AAA-level this year, hitting for both average and power. He's been a prospect worth keeping an eye on for some time already; the only surprise is that he was called up this early.
Playing time: If he keeps hitting, he'll play every day. If he comes down to earth, he'll go to the bench. The Rangers can afford to be patient with Hafner.
Thoughts: I like Hafner, dh-only eligibility and all. The key to his success is patience at the plate and a very good grip on the strike zone. I do expect him to struggle a bit for the rest of the year as he adjusts to big league pitching, but in the long run, the future is bright. He hasn't received a lot of press yet, mainly because Texas had bigger names on its farm teams. If Hafner played, for instance, in the Orioles system, we would have heard much more about him by now.
All in all, don't drop anybody significant to pick him up unless you're building for the future. The dh label hurts his value significantly. If you have a free slot that you can spare, however, I like the idea of taking a gamble on him.