Just booked an excellent fare on Virgin Atlantic from NYC to LHR for this June on Expedia. It was literally $300 cheaper than anyhting I could find on any airline website or other travel site, including Kayak (excluding the really cheap fares on Air India which I decided were not worth it). My final destination is really Paris but with these savings, I can afford a flight over and Eurostar back. So my take is - try everywhere. You never know which site will have the best fare. ^