Technical singletrack, or anything with lots of big roots and rocks would be a bummer, imo, but it sounds like you're handling business. I take my bike on some flat fire road/doubletrack sometimes. Definately good times. It's real fun to get a little blazed and ride on the cliffside trails here, the ocean on one side and rows and rows of brussel sprouts and artichokes on the other. I always forget to let some psi out of my tires, so its usually a pretty harsh ride. Also, 25/28 are ****ty tire sizes for sand or gravel, and my road gear sucks sometimes... when I save up a little more $$$ I am going to build up a old 27" road conversion with 700's and plenty of room for knobby cross tires, two brakes and a low gear for riding in some dirt. I was going to build up an old stumpjumper with horizontal dropouts as a fixed mtb, but it is beat to hell and back.