Yeah, it's a great trail bike. I just got a used 99' KX100 and I love it for the trails. Some nice ideas for taking it on the trails are:
-Get a skid plate. Works Connection has a nice one for $50 and it will save your case from rocks, roots, or anything.
-Downsize your drive sprocket a tooth or two. You lose some speed, but boy will you see an awesome power difference. Wheelies in third gear!
-Get some handguards. Not only do they save your hands, but they also save the bike in a crash. Magnum makes a nice pair for $30.
Ride hard, and have fun!