This service is a god send for many people. On the whole Amazon DVD rental is cheaper, faster and better than other services such as blockbuster as you don't pay any late fees and assuming you sign up to a package suited to your viewing habits then it provides great value for money. I would only rent DVD's from them as it does cost a pound a month extra for games and the whole point to get the most out of the service is to keep on getting, watching and returning DVD's if you are on the unlimited plan. This way you can watch as many DVD's as possible. If you do wish to go for game rental then try and read review beforehand to gauge how long the game would take to finish. If its a few hours then you might as well go ahead because in the end you get to finish the game and save the RRP, otherwise buy the game.
Speed - The average turnaround time is 4 days which is OK although it does depend on the postal service
DVD - The DVD's basically come from lovefilm so they should be fine although it may be hard to get popular titles.

Overall its a great service and although Tesco DVD rental is cheaper, amazon do give vouchers time to time!