I used Sidestep just a couple of days ago to book a flight. I was very impressed with how they work and check the airlines websites as well as the "competition" (Priceline, Travelocity, Orbitz, etc) to get the best fares. When I decided on the best price, times and airline and clicked on it, it took me to the airlines website to purchase the ticket. Basically they just helped me find exactly what I was needing and sent me to the website of the airline I chose.
I haven't used AirfareWatchdog.com before but I heard about this morning and checked out their website. I like the sounds and look of it and hope to use it sometime soon.