I have been using a Helinox Chair One on activations for a while. This was the lightest chair Helinox made at the time of purchase but subsequently they brought out the Chair Zero which is close to half the weight of the Chair One.
Helinox did a 30% reduction offer on Black Friday this year which persuaded me to buy one.
First impressions on arrival is the pack size was significantly smaller than the Chair One. Although both are about 350mm long, the Chair One is about 120mm in diameter while the Chair Zero is about 100mm. This may not seem much less but it represents about a 30% reduction in volume.
An extra you can get for both chairs is a ground sheet, shown below fitted to the Chair Zero.
This is made from strong mesh. The loop in the middle can be used to link it to the lower tube but Iāve never found this necessary.
But the strap can be used to secure everything when packing up. I fold the frame, wrap the chair fabric around the tubes then roll the ground sheet around everything and secure with the strap. It then slides easily into the bag.
On the summits I activate the ground sheet is essential, otherwise the legs just sink into the soft ground. For this reason the weight comparisons below include the ground sheets, which fit in the bag with the chair.
Chair One - 1068g
Chair Zero - 659g
Without the ground sheet I measured the Zero at 516g, which is within a few grams of the claimed weight.
The conclusion from this is the Zero is not half the weight of the Chair One if you are using a ground sheet but there is a useful weight saving of about 400g (or 14 oz).
The two chairs side by side, Zero on the left.
The back of both chairs is at about the same height but the front edge is lower on the Zero. The other thing which springs out from this image are the legs on the Zero are significantly shorter. However, this doesnāt mean the chair is closer to the ground. Here is why.
The tubes supporting the fabric of the chair are significantly longer in the Zero. This is why you sit at about the same height in both chairs. Not exactly the same height, my measurements suggest you will be about 25mm lower in the Zero but without an assistant to help me I could only measure the unstressed fabric. The measurements need to be done again with someone sitting in the chairs. 
A consequence of the shorter legs in the Zero is that the āfootprintā of the Chair is smaller.
Does this make a difference in stability?
My experience of the first activation using the Zero last week is the chair is usable and I could move about in it, to reach the radio or pick something up from the ground without feeling I was about to fall over.
One thing to be aware of is the empty chair will blow over very easily. The Chair One does this and the lighter Zero very readily falls over and on a windy summit you might want to tie it to something like your pack if you want to avoid having to run after it!
In cold weather a foam sit mat is very useful. The thin fabric of the Zero does not provide much insulation.
For those āwider in the beamā there is also a Chair Zero L.
Neither of these chairs can be called cheap and they are hardly an essential extra but for me they do add a degree of comfort to an activation.
