I'm the poster child for gimpy drivers -- I am seriously limited by my oh-so not limber legs and back -- so I can address the issues of comfort and mounting/dismounting from a very personal standpoint.
While even riding in a car makes my back ache and just looking at my legs makes me wince, I am perfectly comfortable in the HyperBike. I do adjust the stirrups out and down for improved angles at hips and knees, but I can drive for hour without needing a break.
To get in, I stand on the left side of the cart, reins in left hand, sometimes resting this hand on the horse's rump. I lift my right leg over the shaft, and as it touches the ground I sit in the seat. Then I lift each foot into the stirrups. Climbing in this way minimizes the moment of vulnerability that occurs with any cart; if the horse were to step forward or spook, you are seated and not in danger of being knocked off your feet.
I agree about the shorter reins. I still have the long ones and have to take time before any drive tucking the ends beneath my legs and seat -- you do not want them dangling and possibly getting caught in the wheels.
Pros:
Fun!
As close as you can get to riding a mini
MUCH more comfortable than an easy entry
Horses LOVE it
Lightweight -- 30 lbs. Easy for even tiny minis. Able to be lifted over logs, rocks or other obstacles
Rugged -- great for driving on trails, rough ground, beach, stream crossings, etc.
Stable and well-balanced. Wide center of gravity makes it nearly impossible to tip
Maneuverable -- we've done pole-bending in ours -- wheeeeeeeee!!
Knocks down and goes together easily without tools -- easy for one person to haul and set-up, and could fit in the tiniest compact car.
My favorite: being so close you can reach out and scritch or pat your horse while driving!
Cons:
Only one person can ride at a time -- no room for passengers
Not a training cart -- get rid of the "green" before you use the HB
Your horse MUST have a solid whoa
Occasionally horses are claustrophobic and can't deal with someone that close -- you'll probably already be aware if your horse is one of them if they hate having you mess with their hindquarters or stand directly behind them.
I would be cautious using the HB with a known kicker, although a kicking/bucking strap helps keep their feet on the ground. (I always have an image of a powerful horse like my 38-inch boy, Mingus, if he were a kicker and we used a kickig strap...instant sling shot!)