I've used Priceline quite a bit with hotels and rental cars and have been very happy with the results, but I will probably never use Priceline for flights. The flexibility you have to give them (your initial flight out can be anytime from 6am to 10pm and you may have one connection) basically makes the first and last day of the trip useless.
For instance, if I wanted to go to NYC and needed to be there by 4pm on 7/21 and could leave anytime after 4pm on 7/24, I would HAVE to fly in on 7/20 and back on 7/25, otherwise I might arrive to late and/or have to leave too early. If I booked direct, I could book a flight out that arrives before 4pm on 7/21 and a flight back that doesn't leave until after 4pm on 7/24. Those two extra nights in NYC would cost $200-$300 or more, so it's highly unlikely I would save enough to make it worthwhile.
Maybe it's not as big of a deal on a full week (or multi-week) trip staying in a timeshare, with a whole family, because that first and last day isn't as important. But from what I've heard from others, there's usually not a whole lot of savings.
Perhaps it might work out for last minute timeshares/flights. Find a great timeshare. Put it on hold. Bid on cheap airfare at Priceline. If you get it, confirm the exchange. If not, release it.