I don't know how useful this post will be, since I don't know if anyone other than four of my friends reads it, but it might make me feel better...
Last spring Larry and I contracted with Boston Green Building to replace nine of our windows. They said it would take a day, and quoted a price (plus time and materials for trim). The estimate had a "10-15% contingency overage clause" which allowed that there might be things out of the contractor's control and costs could go up that much. We chose the company because they're "green" and we want to support that, and the price seemed reasonable.
The actual work took five days, and the final cost was almost 50% over what the original estimate stated. The company's stance is that they informed us that things were taking longer than they thought. Well, duh. The guy was here five days instead of one, so we might have noticed that. What they DIDN'T inform us of was that costs had spiraled so high. Naively, we thought that the work was still within the contingency clause. We thought that BGB would be honest enough to say that they'd hit the estimate ceiling, and they'd hit the clause ceiling, and that costs were going to be considerably over both.
Stay away from this company. We now suspect them of under-representing the time/money it would take so they could get the job. We also suspect them of using our house as a way to up their income: it was never clearly shown to either of us why the job took five days instead of one. All they'd say was, "The house is old and old houses have quirks." Sorry, but this old house and its owners aren't okay with "quirks" that cost us almost 50% more than we were quoted. I'm going to spread the word any way I know how.