We used them for the first time for our May trip in January. We found the best prices there and even on the web sites of the airlines themselves, I could not find comparable prices. They waived the service fee for being first time user, also.
The only problem we had was they don't book more than 4 people at one time. We had five in my family, so I booked four first on the flight we wanted, then immediately went and booked the last ticket separately. When I went to submit the purchase, it popped up that that flight was not available any longer. Well, to say the least, I almost died with that! I called the 1-800 number to see how they could rectify the problem such as a refund on the four already purchased tickets. They said they would, but they also gave it one more try to get one ticket on that flight. They were lucky and I got my fifth ticket purchased with their help.
I called United that afternoon and all of us were on their manafest and they put all our seats together with no problems.
They send e-mails right before the trip to remind you of the trip and also gave us reminders of small changes on the flights.
I would do it again if I found a great deal that I could not pass up on.