Hi
I think Priceline is great...I've booked flights, hotels and car hire with them.
It can be difficult at first to get the bidding right because you can only bid once per scenario.
For example if you wanted a certain size car on a specific date from MCO and your first bid was rejected you would have to change one of the parameters in addition to upping the bid price. Either size of car or date.
I just booked car hire this week for two weeks in June, mid size car for £243 all in, inc insurance and everything. This saved me about 60-70 pounds on Sunchase prices which are generally really low.
Don't use the american site to book....like others said it does not include the dreaded and very expensive insurance.
Also don't be put off when Priceline tell you your bid has poor chance of success, give it a go anyway if it's your first bid.
I think Priceline uk only comes up with Alamo cars so if you really hate Alamo (and some do) it's not the best option.
Hope this helps
Karyn