Its a good site, I used it a couple of years ago when I had to re-take General Nav and didn't want to pay 50 quid to Bristol just for one exam! The Gen Nav database was great and got me through no probs (92%).. dunno about the other subjects though.
Regarding explanations, I had difficulty with a couple of the distance / time etc questions (still do to be honest!) so I emailed them and got a couple of good explanations, which really helped.