The Infernalist is a very interesting class. They have great potential for roleplaying, both as a villain and as a PC. The layout is very nice and follows the same formula as the other class books, and the additional feats are quite nice. I could easily see additional classes like this, but based upon other otherworldly powers but changing the automatic feat gained at forth level as well as perhaps the penalty gained (levels of the Tainted flaw) as their level increases.
Personally, I think it would depend on my character's motivation as to whether I'd work on countering the flaw or improving my strengths if I were to play as an Infernalist. If I made the pact out of greed, I'd not worry about the drawback, but if I made the pact out of desperation, I'd probably be horrified at what it was doing to me and fight against that as much as possible. Of course the drawback only really hits full force if you are playing with the Corrupting Magic Campaign Quality, or you have some additional way to gain Tainted levels. But the potential power in the class is hard to ignore in either case.
[5 of 5 Stars!]