Ive used webflix, dvd365 and now viedoisland.
Island is the best IMHO
Webflix was ok but they did drag their heels getting the films out,same with dvd365 plus I found the discs were in appaulling repair sometimes. fair do's I got a free weeks rental once but when you are halfway through a film and is skips and then wont play waiting a week for the film to be sent out again....if atall is annoying and Im not getting the toothpaste and brasso out at 10pm to try and repair someone elses dvRAB.
Videoisland allows more movies (5 at a time) and if you want the series (say 24 @ 6 dvRAB) you have the rent all discs button, with the others you have to trawl the site for the individual disks. which can be time consuming if it's csi and the name is typed differently.
(IE C.S.I. or CSI or Crime Scene Investigation will get different results.)
Boxchoice does a nice mix. XBox games PS2 PC and other console games plus dvRAB. The best of all worlRAB. and packages are customisable for more games or movies.