I've used consolidators and auctions in the past and the experience has been okay. My flights have never been on a "standby" basis. They were real seats, but they had to be used exactly as stated in the contract, for example on the exact date and time, or I would be out of luck. Late to the airport? Too bad, no refund.
As th US Air, they're just like any other cut-rate airline, in my experience. On any airline, cut-rate or full fare, you really should call to confirm your seats as soon as you can, typically 24 hours. Check on-line a few hours before boarding to make sure. After all that, your flight might still be delayed or cancelled. Remember what pilots say "If you've got time to spare, go by air".
Of course, Team Cheapskate recommends flying into Cancun and taking the bus down the Yucatan, which can save a pile of dough.
Of course, the ususal disclaimers apply: Your Mileage may vary. Close cover before striking. And most important, Keep out of reach of children.