I always want to read all forum posts, so I use <a class="btn">Mark Forums Read</a> quite a lot.
What I've been doing is first reading all new threads, then pressing that button. I am sure that sometimes (by mistake) I don't read everything, but I still press the button to have all other threads that I had read marked, instead of opening all again.
I think it would be better if we could only mark a whole sub-forum or even only a thread as read, so we could keep reading without bypassing one thread by mistake.