My experiences have been similar to ringingcod.

If I call them for a car in the afternoon or evening, it's generally pretty good.

On the two occasions where I pre-booked a car before 6am (despite giving them several days notice), they let me down. Despite on the second occasion, me calling them the night before to check that all was OK and it was definitely in their books. Both times, they didn't contact me. I phoned them 5 minutes after the pick up time for them to tell me that they were going to be 30-45 minutes late.