Fine for an hour at a time, then starts to get on your nerves as it is not easy to stand up, you have no real way of moving yourself around as your legs are folded up and it does your knees in.
I had back problems and tried it at work. Ended up giving it away.
The best thing i know is an height adjustable desk. All the Designers who do CAD work or graphic design use them. Stand up for an hour, sit for an hour etc.