I have had my Sitpack for some time now so feel qualified to write a review about it.
I originally bought my Sitpack as we were going to a concert and I was bothered about standing up for a long time - I'm disabled and can't stand for a great length of time. I thought that it would help me, which it did.
Firstly, I was surprised at how small it was - about the size of a can of Coke. (I soon realised that I should have ordered the neoprene case with it so that arrived shortly afterwards). It is made of plastic which, I felt, probably wouldn't withstand my weight. After having a good few practice sits on it (making sure that each joint was locked by turning there stem tightly) I used it at the concert - Ed Sheeran in Galway, Ireland, just in case you were interested - and it worked a treat. I used it to sit on for about 25-30 minutes whenever I felt the need, putting it away in-between, which really isn't a problem and takes seconds.
I've been using my Sitpack regularly ever since. It's certainly a step up from a shooting stick which I used before as it is much more convenient and sits nicely under my jacket using the neoprene seat cover and strap.
Overall verdict - it's brilliant! I would never have thought that such a small piece of plastic could turn out to be such a clever piece of engineering. It goes everywhere with me in the back of my car, carried on my shoulder, and is then used when there are no seats available when I'm on my travels.
I'm quite tempted by the new seat - if it's better than my trusty Sitpack then it'll be outstanding - but I can't recommend my Sitpack enough!