I tried Amazon once and had no prob, but that was when they limited you to 6 discs a month so we quit. I plan on going back soon. I'm currently with Blockbuster, and the discs are scratched sometimes (that's unavoidable with as many people's hands as they go through), but none to the point that it impaired viewing at all. I'm going to leave them because it's gotten to the point that they don't send us new releases (all the dvd companies do this if you're a high volume user).
Just avoid Lovefilm, as they not only have the crappiest discs around, they don't let you rank by number the discs you want and they make it very hard to quit! Be careful, as Lovefilm is the actual company that handles many of the other home DVD providers, like Tesco and easycinema.
Here is a list of reviews and costs of various DVD rental services. It's only slightly out of date, as it hasn't noted that Amazon now has unlimited rental plans.
http://www.uk-dvd-rental-guide.com/.