I subscribed for a while when I couldn't find an answer to a question anywhere else. Very good for focussed discussion boards, e.g. Volvo 2000 series engines.
There are some professional mechanics hanging out who tend to have good answers and then tell you they can source the parts. The board is more US oriented, so you get some experiences from boat-engine combinations that you would not find in the UK. And they all seem a bit more experienced stripping down their engines than we generally are this side of the pond (unless we just don't brag about it so much??)
You can't read the answers to the posts unless you pay.
So to summarise it: probably worth it if you have a specific engine/gearbox problem you are trying to solve, but I did not renew after a year.