I use Priceline when the price gets out of whack at the rental car company sites. I usually visit the same place once a month. Sometimes the rental cars can be had for $20 or $25 per day and other times its $50 to $65 per day. It doesn't seem like a supply and demand situation but who can tell.

If my plans change and I had already Pricelined my car, then I would be out of luck.

I end up using PL when the pricing discomfort is too much.