I first became aware of this product when I was reading Birdlife, the magazine of the RSPB. I have never seen it advertised in any art magazine or by any art supplier. I followed up the ad and found descriptions on the net, together with a few sources of supply, all mail order. Efforts to find a local supplier, so that I could actually examine the product, came to nought. After mulling this over for a while I bought one.
The Walkstool comes in two versions. There is the original known as the Comfort 45/55/65/75 - the 45 is the smallest and the 75 the largest. The other version is a cheaper budget one made in China called the Basic 50 and 60, two sizes. The numbers denote the seat size in centimetres. I have the smallest Comfort 45 which I find adequate. This comes with a nice canvas case, 14" long and about 4" wide, well-made and with a carrying strap. It fits in my backpack with all the other stuff. Incredibly the Walkstool weighs only 18ozs and yet will support up to 550 lbs! As I weigh about 150 lbs I haven't tested this claim but my friend Mick Carney has since bought one, based on trying mine at Urchfont, and he is a much bigger chap.
Any downsides? Price between £50-65, depending on size, with the Basic model about half that. I found the instructions clear and the stool easy to assemble and pack up. However you do need to exercise care in doing this and the instructions point out a possible problem if not done properly. This involves the telescopic legs. Nevertheless I've now used mine on several occasions and am very pleased with it. Where possible I prefer to use my director's chair, the one with the side shelf and pockets, but this only if I'm painting very close to the car. For portability and ease of use the Walkstool takes some beating. There are two or three people selling this product, either on the web or mail order, and mine came from Uttings in East Anglia http://uttings.com/ just type in walkstool in the appropriate search window.