I was into it big time for majority of my life....even did it in highschool, was third in new england that first year I tried it. Started to get in the way of horses though, so I ended up choosing horses over biking, sold all my bikes, and havent done it since....what a blast, and exercise it was though!
I really enjoy trail and path bicycling, but not true mountain biking (fling your body on a piece of moving aluminum down a hillside).
Since moving to SW MO, I find I do very little of it however. It is very hilly and steep here so it is more like mountain biking instead of woods trail riding. Also, the summer heat is miserable! I can barely breathe outside in the humidity and sunshine it is so hot. By the time I can deal with the actually exertion, it is dark and dangerous.
Mountain bike and horse ride here. At my back door there is HUGE trails network, the ski hill in summer is mountain biking and we are also the start of the Trans Rockies mountain bike race.Heck there is even trails that will take me out to my horses in about a 45 minutes ride.