I LOOOOOOOVE driving my minis :) I haven't done a whole lot of AMHA/AMHR driving other than obstacle driving, because my two best driving horses aren't registered, but I'm hoping to with one of my stallions once he's trained.
I do compete in pleasure driving shows, I go trail driving, driven in parades (including the Rose Bowl parade these past two years), and am part of a miniature horse driving drill team (www.minimystique.org
But my main love is combined driving, there is NO greater thrill to me :) My mini Chili Pepper and I fly through the cones and hazards and just have a BLAST :) And with the exception of a test or two, I like the dressage as well (there is a certain prelim test that always gets me! Argh ;) )
My husband also does combined driving and is part of the drill team as well :) Can't get him to go riding with me, but he enjoys driving the minis!