I have purchased and used about 75 Skyauction vacation certificates over the years and love them. They have changed my life.
I usually wait for a sale and pay less than the $249 price. Two years ago, I bought 8 at $149/certificate - the lowest price ever. I have snapped up many at $199 and have seldom paid more than that. They go on sale a few times a year for different amounts - $149, $179, $199, $219, $229 or $249. It is impossible to know what sales will come up or what the lowest price will be. Prices have been creeping up the last 1.5 years.
Only get them if you can travel off season and don't mind a one bedroom unit. Bigger units are rarely listed in inventory except for the very slowest seasons.
Inventory changes all the time and it is possible that a unit may not be available for the specific weeks you want. Check todays inventory at resortcerts.com to give yourself an idea what timeshares show up in which months.
Make sure to read reviews on the resorts and unit sizes so you have a better idea of what you are getting into. Some resorts are much nicer than others. Some one bedroom units can be very small without some amenities like full kitchens, balconies, spa tubs, etc.
I love using the certificates for Orlando, Vegas, Sedona, Williamsburg, Outer Banks, Myrtle Beach, Durango and Pagosa Springs.
Reservations cannot be cancelled or changed so keep this in mind. The more timeshares in a certain area, the better the chance of having excess inventory show up out of season.
It has become a bit harder to get good units recently. If I can't get the week and resort I want using the certificates, I book a specific week at a specific resort using Skyauction when they have a sale and only charge $199 in fees for a week instead of their normal $299. My winnng bid is usually $1, so the week costs me $220 ($1 bid plus $199 taxes plus $20 processing charge).
If I strike out with Skyauction, I have found some great deals by using Ebay.
Let me know if you have any more questions.