Never had any issue with any recognised car rental company - Use Hertz regularly for work, used Europcar, Avis, Alamo, National, Budget - all fine. Where are you renting? If in Europe then Holiday Autos can have good rates at the normal holiday places. I have booked through Ebookers and expedia previously too. I usually book the cheapest for wherever I am heading - as long as its with one of the big companies mentioned above.