Every since we traveled to Salt Lake City on Delta, and found the plane that the plane was oversold. There was a family ahead of us in line that had bought their tickets on priceline. Instead of getting 'Y' class tickets, their tickets were 'bumpable'. They were garranteed to get back home, but according to the fine print in their contract with priceline, they could have to wait up to 24 (or 48?) hours.
We had used expedia.com (owned by Microsoft) and paid EXACTLY what they did, and we had 'Y' class tickets. I felt very sorry for them, but I have never had an issue with expedia. I can cancel my tickets, or change them without a problem.
For what it's worth, Expedia.com has been the most consistent and best site I have used.