Don't know about ATP but have heard nothing but bad things about Sierra. There were 2 guys at my flight school who switched from Sierra and they said that they were being gouged for money. Both were very sharp guys and Sierra had them doing 70 hours before they got their private. At an FBO, I guess I'd accept that, but not at a 141 school.
I had an instructor that came from ATP and he liked it a lot. However, it is a crash course. It requires total dedication b/c it moves fast. He got his instrument, commercial, multi, cfi, and cfii in 103 days.
I'll second what flyboy said. I did the program at ATPs about 2 years ago and finished up all my ratings in 95 days. I'd recommend it over any other program out there, if you don't mind the very fast paced kind of flight training. There's nothing fancy about ATPs, or their program, or their locations, however, they do deliver on what they promise, and they won't raise the prices or charge you any hidden costs during your program.