i use amazon as i was too lazy to bother looking at the others. you could probably use the introductory offers from all the various places offering these to rent dvds for a year for virtually no money at all.
amazon's is fine. excellent selection, fast turn around, and normally i get what's at the top of my list. £8 for 4 dvds a month, you get 2 at a time and there are no deadlines. i did have £10 for 6 a month (3 a at a time) but found i didn't always have the time to watch more than 1 a week.