The Cheapoair site allowed us to book on two different airlines with not enough time in between. We didn't know at the time that the two airlines had no baggage agreement, so this meant we would have to collect our bags in between flights, check in at the counter at the 2nd airline, and go through security again, which for an international flight (LAS-->YVR) must be done at least an hour in advance.
Our time between flights was 1:04. Air Canada told us this was an illegal booking. I also found that when I spoke to their reps on the phone they were completely unhelpful.
Forget about checking your bags all the way through, checking in online or making seat assignments. Best to be avoided.
Review about: Cheapoair Flight Booking.