I really enjoy ground driving, and find it a great tool. When I was doing my horse courses, we did it during our later lunging classes, and when we were breaking in youngsters. I found that it was a great way to get a feel for the horse, and for the horse to get a feel for your hands. The only thing I don't like is when the horse is a go-go thoroughbred and tries to canter straight through your hands. I basically went skiing through the arena behind him, and eventually got him stopped with a one rein stop.
I also used it to do mouthing, and it worked wonders, especially when my mare decided that bit pressure = cue to rear and almost flip. I'm so glad that that happened whilst I was on the ground rather than in the saddle.
I think it's an invaluable tool and it can be fun, especially if, like Lins said, you take them through an obstacle course. I had my mare winding around barrels and going over wooden bridges in the arena, and it was a great experience for both of us.