We used Avis through Priceline last week...not a great experience. First, be prepared to provide your personal auto insurance information if declining their insurance (even though nothing in the documentation suggests you'll need this). Also, Avis "kindly upgraded" our standard vehicle to a premium vehicle because there weren't any standard size available. We got to the assigned spot in the garage and found this big, old, dirty Monte Carlo. Off to the Avis booth in the garage to complain and then a standard size vehicle was magically available. Normally we're heading out of MCO in our rental car within 30 minutes of landing...
Alamo/National/
Budget...we've used them all, we've used Priceline before too with much better results. Just know what you need, what you rented and what
not to settle for and you'll be fine.