I havent ridden them since the registry has started, but the ole mtn horses we use to call them, is what they are.
Like any breed there is good and bad. Overall they are great horses. Usually level headed and calm. Typically are sure footed on the trails, good for an all day ride and some can really rack. I just got back into horses and was wanting either a TWH or a Ky Mtn. Found the Walker I wanted before the Mtn horse.
They are typically versatile for work, trail rides or driving. I was told that the breed was originally developed over the years due to a farmer in the mtns needing a versatile horse that could work on the farm, pull the family to town on the wagon and had a good saddle gait so they could travel long distances without the wear and tear on the body.
The Mtn and Rockies are a mixture horse. Kind of a mutt usually crossess of Morgan/TWH/Saddlebred/Qh etc. bred out for the trait they were locking in onto, which was a good saddle gait. Years ago they were a dime a dozen, glad to see them get their dues and be the breed of choice as some of the best trail rides I took as a High Schooler was straddled on the back of one.
Im no expert but this is my experiences with them.