It's somewhat awkward/clumsy and makes controlling standard games more difficult. Sure, in "Super Swipe Dart Throwing!" you'll have better control because of the touch screen, but no one would ever choose it over a PS3/360 controller for something like Call of Duty.
Well, coming from a PC background (CS, Q3 etc), playing FPS games is already extremely awkward on the PS3/360 controllers, so I don't really think that's a great example. Actually, I'd rather use the touch screen like a giant track pad to aim in an FPS (if I can move and fire with the other hand). I also think it adds a lot to managing inventory, switching weapons etc. I haven't tried it though, so could be more awkward than it looks.
