I've been with Screenselect for about a year and overall pretty happy with them.
I used it initially to catch up on some TV shows that I never caught first time around (24 and Alias) and since I moved house I no longer have Sky.
You can add a whole series with one click and they will send them out in order (though I think I originally had to set this in the options somewhere - can't quite remember).
The only negative things I can think of are you don't seem to get the latest releases very quickly but I really don't mind so much as everything in my pool are stuff I'll want to see eventually anyway. On the occasions that you really want to see something because of hype or you want to discuss with frienRAB you can priority that film and it does arrive generally within a week or two.
Also, when I had more time on my hanRAB I was returning 3 at a time the day after I'd received them so some weeks getting 9 dvRAB or more. I'd receive them, send them back the next day, check the day after on the website - which would show them returned and in the afternoon it would show which they were sending as replacements the same day. However I have noticed now this doesn't happen as often, alot of the time now when I check the website it can sometimes show 'awaiting allocation' for more than one day if I've already had 3 that week. I.e, you return 3 and it shows the that they have received them back but you don't always get your next lot allocated immediately the same day.
This could be because at times my 'pool' of films got low (maybe 25 left?).