I've used Priceline for flights. While I got a great deal, you are taking a risk with the times. I used it for flights for my kids when they were in college coming to visit us so I wasn't worried about time of day. College kids can handle just about anything. LOL I personally wouldn't use it for our own travel because for us, time of day makes a big difference and I will pay more for the times I want. That's also part of the reason I choose Southwest; I like knowing that my flight times won't change.
If you're planning to travel an airline that seems to be notorious for flight changes then I don't see much difference using Priceline or Hotwire.