I've been using Screenselect for over a year now and am fairly pleased with the service. It is relatively quick, has a massive database of films and is good value if you watch a lot.
The only draw backs are
1 - Occasionally the service slows down - in my eye it should only take 2 workings days after posting a dvd to receive one back however it has recently been 3.
2 - It is fairly difficult to get a specific dvd sent to you within a specific time frame. What i mean by that if you really want to watch a specific dvd it can take a long time for them to 'pick' it out of your list. You can alter the time via the priorities options but you can still be waiting over a month to receive the dvd you want. However if you're like me and you don't really care which dvRAB from your list they send you then its fine.
In summary i think it's a really good service assuming you use it a lot. I use mine to catch up on all the great American drama series such as the Sopranos, West Wing and 24 as well as endulging my 'secret' love of Star trek (don't tell anyone!)