I have bought from them and love it. I have only ever used 1 certificate, but had no problem using it all all. However, they never expire. If the restaurant goes out of business or quits participating, you can trade that certificate for another.
I would offer a few pieces of advice:
Pay attention to the minimum purchase requirement before you decide to buy. A lot of $25 certificates are $35 mp, but some are $50, which is usually too much for me as there are only 2 of us and I would still have to pay at least $25.
Check out the menu and make certain it is a place you want to eat.
Never buy at full price, even though it is usually just $10 for a $25 certificate. They regularly run specials for 60%-90% off. Right now they are offering 70% off with the promo code SNOW, so you can get a $25 certificate for just $3.
The $300 for $25 sounds like a great deal, if you can purchase them in the increments that are best for you.