I had a terrible experience with Expedia. The absolute worst customer service I've ever seen. That's coming from someone who works in a similar industry. Was supposed to travel to DC when they had that major snow storm back in December 09. Had it booked for months. Expedia took my money but refused to help me find another flight. Actually had them hang up on me twice. Big no-no, and I wasn't even being rude to them. Had to deal with Delta myself.
Hotels.com is pretty good. Booked my last vacation with them. For big/international trips, I'd go with an actual travel agent, or book everything direct. Not through these online all-in-one services.