I've had the forerunner to the Panther for many years now and find it really good, bought mine from Reeves before Bushwear started selling Panthers. (
BushWear: BushWear 'Panther' Folding Highseat - £299 offer on the full kit: seat, extension and legs, been this price for a long time now)
I take the comment on the seat hook but you'd have to give it some stick to break it, I did get a second bracket for use with the freestanding legs kit.
One BIG, BIG advantage over the other seat is that the seat bears on the tree at shoulder level meaning your centre of gravity is kept low and I only use a tree strap with the extensions on, the X3MI seat looks inherently unstable with the tree bracket at bum height! Now sitting on it rather than in it would make me nervous of moving!! I've stood on the seat of mine to trim some branches above it, don't think I'd do that on the X3Mi one.
The trick is to make sure the 'teeth' of the tree bracket actually make contact on both sides and the feet ar square to avoid the 'wobble' when you move, although this isn't desperate and soon over... just teaches you to move carefully! I'll tie it to the trunk if i can't get a solid contact though.
I also carry a stick to put across the rails to act as a brace and also avoid leaning too far forward (ie minimal movement). To take a shot at your left or right limit I put a foot down a rung allowing me to swivel and take what is effectively a standing shot off the rails. (A quick look at the other seat suggests this might not be so easy with the angled rail)
As for portability..... seat over one shoulder, rifle over the other, stick, foxpro etc etc. Have also carried the extensions and the legs on occasion.
Good for me.
atb
FBW