I use TXU. I have for over a long time. The main reason is average billing. But I also get the perks like the cash back and special offers. Average billing kept me from having an $800 electric bill a couple of years ago before we replaced out AC units.
Just switched to their 100% wind energy plan. Its a bit more expensive but I can shove the noses of my snooty neighbors in it, who rave about their Hybrid saving the planet when their kids have the AC turned way up while leaving the doors open on the house.