I think there's also a random factor involved. I remember when I was a lurker, there would periods of up to a week where I couldn't browse the forums, and then it would be open again for a while.
Either way, it's only a $10 one-time fee. However I think the general forums aren't really worth it, and most of the specific ones are a bit so-so. The signal/noise-ratio in GBS in particular is horrendous, and most of the more specific forums tend to move rather slowly, but they're pretty good other than that.
However, the automotive forums (Automotive Insanity and Cycle Asylum) are the best damn car/bike-related forums I have ever used. Very little bullshit, some very knowledgeable users (one guy is like a Jeep whisperer, he knows EVERYTHING about them), and interesting projects. There's no muscle car/import hate, no EV hate, no macho street race posturing etc., and there is absolutely no common thread in which cars/bikes are represented, apart from the fact they all have wheels (a couple may have tracks, though).