I haven't. I'd do some web research maybe and see what people have to say. All I can say is be cautious. Not so much like they are scammers but read the fine print very well! While some companies like Travelocity, etc., offer good fares, many of these tickets have a lot of restrictions.
What kind of fares are they offering? And what is the comparison to airline websites and other travel websites??
I think in theory it's great but sometimes customer service and flexibility is lacking.
(airline employee here so I've heard the horror stories!)