Wouldn't accept my payment.
Cons: When I first arrived, the rental location had no cars available and the attendant had to argue on the phone with someone to get them to drop off a car. While the car was being delivered I was told that my credit card is an unacceptable form of payment because they can't put a hold on it for liability reasons since it is "prepaid." Kayak accepted my credit card. I was charged by the insurance provider for the rental insurance. Insurance was purchased so I'm not clear what liability he was referring to. This stranded me at the agency. Luckily I got a ride home from agency car delivery person, but his attitude was sub-par because he had to go out of his way to help a customer who just got stranded 10 miles from home.
I must say that this is the last time that I will ever use Kayak for anything. If they can't verify accepted payment types during the transaction then it's not a service that I'll ever use again. I don't think that inconvenient begins to explain how much this messed up my travel plans.