We have used it for our last two trips to Orlando. Worked out great. Ended up saving a decent amount of money. First time we got Budget (you cant pick company)...that was very easy, right at MCO.....an economy car ended up being a Ford Escape...so plenty of room for two adults, luggage and golf clubs.
Second time was Hertz, offsite so it took more time to get the car, but still no hassle.
This is what I would recommend.....shop around and make a reservation at the lowest price you can get for the car you want. Then, make a bid on Priceline for same car, just a lower price. If accepted you get a deal and can cancel the other reservation. If not, you still have the reservation you made....you can also wait and do this close to when you are going. Sometimes I think you get better deals closer to the date you need it because they know they are less likely to rent the cars at full rate. If you book way out, they might not be willing to budge on the price.