I have used one and I didn't like it at all (it was on a clients computer that I often had to use).
The main issue I think I had with it was an issue of precision. Using your forefingers for buttons and small movements with you hand in the horizontal position is more natural to me. My thumb is just not as fast at clicking etc... and small movements with hand in vertical position are not so easy. That vertical hand position, imho, is more sorted to analog feedback type jobs, such as when you use a joystick in a helicopter simulator. Think about the hand position of a doctor using a scalpel compared to a farmer gutting a pig.
p.s. it has been a struggle to say hand position so many times without making a smutty comment - but if you do want another comparison, think about the vertical hand for yourself or the "more delicate" horizontal hand for her