I've never used them for airfare because you're plans need to be REALLY flexible since as you said, you can't pick the times. The other negative is that you agree to at least one connection. So the absolute worst scenario would be:
Late departure starting your trip (8pm?), a 3 hour layover at another airport, and then continuing to Orlando. On the way home, a 6am flight, a 3 hour layover at another airport, then continuing to your final destination.
The above scenario is probably extreme, although entirely possible. You'd not only lose a day on the arrival side of things AND a day on the departure side of things (due to LATE timescheule at beginning of the trip AND early timeschedule on the end of the trip), but you'd also spend a lot of time traveling compared to booking direct flights.
On the other hand, i use them all the time for HOTELS and find it to be great.