It depends. Priceline offers traditional car rentals where you don't have to pay until you get to the counter. They also offer various flavors of pre-payment options, some which lock you in from the get go. Here's a quick breakdown:

Pay at Counter - No need to enter payment information up-front. No change or cancel fees obviously since they don't have your payment info. Know what vehicle and company you are getting up-front.
Pay Now with rental company - You pay up-front to the car rental company. Same as a "pay now" deal at Hertz, Avis, etc. Know what company/vehicle you are getting You may be able to change/cancel, but with some restrictions and/or a penalty.
Express Deals - You pay a known up-front price and you find out which vehicle and company you get once you provide payment info. No cancellations or changes allowed in most cases. It's use it or lose it.
Name Your Own Price - You bid how much you are willing yo pay and then find out if your price is accepted. You find out which vehicle and company you get once you provide payment info. No cancellations or changes allowed in most cases. It's use it or lose it.

Generally the cars are pretty much the same between the different options. They don't typically try to give you a crummy car if you book through Priceline (or elsewhere). You'll know up-front if it's limited or unlimited miles before you book any of the options above. You'll also know the location (airport or local) ahead of time.

Pickup and return times don't have to be exact, but rentals are based on a 24-hour day no matter what option you book, so if you're late to return and it puts you into another day, you'll pay regardless of which option you initially chose.

You can also get a quote from AutoSlash by using the links below. We refer customers to Priceline, but at discounted rates based on your memberships and publicly available discounts and coupons. Often you pay the same or less as the prepay rate, but on a pay-at-the-counter deal.