No, I haven't used them but out of curiosity I checked my airfare on Kayak, Webjet and Justairticket, which I is the site I used. Justairticket was $200 less than Kayak and $100 less than Webjet. The downside is that they don't have deals on multi-city flights (open jaw). I was nervous about using an unknown company, but I confirmed my ticket with American Airlines and they had my record before my credit card was even dinged.
I've read reviews that some of these companies may not be helpful post purchase if changes are necessary but I always buy the travel insurance they offer at the time of purchase. That way, in case of missed ground travel connections or accident, I'm protected (or at least like to believe I am - never had to use it... Knock on wood)