I have used Boomerang for a while now and don't really get a huge amount of play time for my games. So i think £3.49 for one game a month is pretty good value, but they do have one that gives you two games a month for £7.99 as well as unlimited options.
Can't find any other rental websites that are cheaper.
My experience with them has been great, although I've never gone after the brand new games. Although i know there is a priority subscription for that if you like. Turn around time is quick, and you can select how they get dispatched to you to speed this up.
i.e. You can select 'Quick' turnaround and they will send you anything on your list. Or you can select 'Top 3' and only get one of the top 3 in your list but you might have to wait.
I'd say for the more casual gamer, this is the best deal around for rentals.