We used to use it quite a bit, and I thought it was really good. The only thing I got frustrated with (as mentioned above) is that you can bet that when you REALLY need to use it, it'll be booked out already. I think people tend to book quite a bit in advance.
The other thing was that you tend to book for the minimum time you need, and then if you get stuck in a jam or spend longer going to B+Q than you thought it ends up being a panic to get back!
Apart from those two points I thought it was really easy to use, simple to book and quite cheap. Only reason we stopped using is when we bought a car..
hope that helps