I'm with Amazon, can't really fault the service they provide, when I send the dvd back I get an email confirming they recieved it then before I know it the next one arrives.
My only complaint is not with Amazon but other people who use the service who manage to scratch and leave fingerprints all over the dvd's. I think that must the most comman problem with all dvd rentals but on the plus side only one disc I've rented was unplayable and Amazon sent me another copy without a problem.