I've used them, especially for expensive overseas trips, and I was happy. You can get good deals. I usually look for the cheapest advertised fare I can find - you know, the ones with few seats if you book six months in advance in the off-season for blind senior-citizens who are members of their discount club - and offer that. They've always taken my offer, since its probably a good estimate of what the airlines think it actually costs them to fly.
However, if you are picky about time, schedule, route, etc be warned. They promise you a flight in a 24 hour period of when you request. Most of mine have ended up with just about the worst schedule available. Leaving early in the morning, layovers, and such. Remember, these are the tickets they are having trouble selling normally. Its fine with me, I'm perverse and enjoy exploring new airports, but it might not be for you.
David