Yeah, it's a lot of fun...and definitely a great way to improve your bike handling skills. After negotiating a steep muddy or technical descent on a rigid bike with drop bars and skinny tires, almost anything seems easy on a mountain bike with suspension. I like riding trails on both types of bikes, because one makes me appreciate the other.

As far as obstacles are concerned, you have to decide whether it's faster to ride or run. Usually riding is faster, so you see more and more people these days bunny hopping the barriers (and more course designers trying to prevent that). Sometimes though, you get a really steep hill like the one in the second to last picture that is generally faster to run up. It's always fun though when you can ride it and pass people who are pushing their bikes.