we used alamo, with Virgin holidays, no hassle mint service, picked a car from the line they offered (fiat 500 LOL) felt like a dwarf on the roads, beat a few camaros off the line at lights too!
when booking you need to secure it against a credit card as a deposit which they check when you leave the pick up area so make sure you take the credit card with you that you booked against.
found driving in florida a piece of cake to be honest, right hand turns at junctions when the lights are on red are a bit strange at first but suprisingly it works very well.
enjoy your holiday, you will have a great time!