I doubt I can put it better than
Mithadan, but....
This is the only Tolkien forum (or any forum) that I post on. Really, and especially all those years ago when I found this place, being an active part of such a community was contrary to my generally private, quiet nature. I found the Downs when it was part of the EZ Board family. I think there was one other Tolkien site I joined at the same time, but the fact that I can't recall the name indicates its importance to me now.
I was highly impressed by the breadth of Tolkien knowledge on rampant display here, not to mention the general civility of tone that was rigidly enforced, and was hooked from the start. I never had any desire to visit other Tolkien forums later, even when the movie-hype reached its zenith, and I found myself in the decided minority opposed to the movies.
Today, when the number of forums is fewer, I feel even less need to shop around.