I love my hyperbike. As the manufacturer states on his site, they are not meant to be used for starting horses, as they are harder to make a quick exit from than an easy entry, though I cannot imagine that it would be much more difficult than a show cart with a basket (I haven't driven one, so don't know for sure). They are very stable due to their wide wheel base and are therefore much safer on uneven teraine as they are difficult to flip. My horse was in a driving accident recently, she was hooked to the Hyperbike and ended up loose with the cart attached. I am so glad that she was hooked to the 'bike, as had she been hitched to a vehicle with a higher center of gravity, narrow wheel base, and longer shafts, it is very likley that it would have flipped on her and caused much more damage to horse and cart. As it was, both escaped relativley unscathed. Plus, they are a lot of fun!
I do not like the look of the dorsal hitch carts, they do not look safe to me at all...