Pretty useful
It's a little wobbly when you're typing (though, in my experience, less so than the super expensive standing desks they have in offices). Obviously the key down side is that this is not easy to put up and take down, so you tend to end up standing most or all of the day, and actually start rather looking forward to your breaks sitting down. Maybe that's a good thing?
At the end of the day, it's a great 'value' option.