My first trip over here I was booking two separate roundtrip transatlantic flights on two hours sleep. Consequently, I was so busy trying to lock in a decent fare for Lene on Orbitz (since Expedia wanted 2.5X as much for the same seat on the same flight) that I completely forgot about the Expedia fare I'd already found for me for about $500 less.

The moral seems to be that 95% of the time you'll find the same basic fares on Expedia, Orbitz, Travelocity and Priceline, but since the other 5% of the time looking at all of them can save hundreds (or thousands) of dollars, it's worth a look. In my recent but significant experience, either Orbitz or Expedia (and usually both) will beat everyone elses fares. As a rule, Expedia has given me the best deals, but occasionally Orbitz beats them; the case referenced above is the only time the difference has been more than about $20 though. Good luck and safe travels.