I recently had my main roof replaced this past Fall. I called numerous contractors for an estimate, including Bob Santini, Crest, United, and Schulte.
First of all, Bob Santini is the coolest guy in the universe. I called him asking for a quote. He declined, because he felt that other contractors in the area would do a better job on a flat roof. He gave me what he thought were the best candidates for the job and the best material of construction (an endless debate for flat roofs). Also some negotiation pointers
I've used Schulte before on a smaller roof, and they provided an estimate last time (did not end up using them, though). When I used them, they did really great work. Generally recommended.
We used United this last time around, based on Bob's rec and a competive bid. Very, very satisfied. I've had other, non-roofing, contractors comment on how nice the roof is. Give them a shot at an estimate.
Of the four, Crest is the only one I didn't have a great experience with. After much cajoling on my part, I never even got an estimate from them. It put my project back weeks. I understand these contractors are busy, but at least send me a "thanks, but too busy for more jobs right now" message. *shrugs*