I have one. I used it pretty hard for a couple years. I'm not a fan. It works in a pinch, or for shorter hunts, but it's not comfortable for me for long periods. I did some all day sits in mine, and they were miserable.
One big thing I don't like about it is the inability to add bibs while you're in the saddle. I frequently have a 1/2 mile or more walk to stand sites, usually up a ridge or two, and I overheat on the way in, and it will take me an hour or so to cool off, which is when I'll put on my bibs and heavier layers. That is impossible in a tree-saddle.
You also pretty much need to wear kneepads or your knees get pretty sore (or at least mine did). And then my feet start giving me trouble because they're just resting on treesteps for long periods of time. (maybe I'm just a wuss)
I still have mine. And won't get rid of it. But I prefer a hang on stand.
They are quiet. They do offer 360 degree shooting. Shooting from it has been the hardest part for me to get used to, particularly the drop shot. But like anything if you practice you pick it up.
If you are using your gear properly they are theoretically safer because they can completely eliminate fall factor, there shouldn't be slack in your system.
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Alot of our areas are small..... as small as 5 acres. Any pressure just wandering around and these axis are gone. They sense any pressure they are gone. Trees are pretty much all you have in those cases. Hunt the edges and hope for a mistake. Saddles are the prefect answer. Probably killed half my deer last year out of a stand.... all my stands are saddles.
Always use knee pads. Tree limbs and pins for your feet are the key. We don't do 12 hour sits here but 4 to 6 hours aren't hard. I find myself falling asleep in it all the time.... Love the flexibility and the athletic nature of them. In reality when set up right you are actually "hiding" behind the tree for your shot rather than stuck out in mid air in front of it. In several years I have missed two shots from it. One was on me and a brain fart on yardage..... the other was a major string jump..... these axis are string jumpin' sons for sure..... the straps you can actually use in many cases as a "rest" to shoot from. Can lean into a shot from near any angle with no fear of falling.
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I bought my tree saddle in 08... comfort was a major concern after suffering a vertebrae fracture in my L5 a few years prior. I have had no comfort issues. However like some have mentioned already, they take some getting used to, and even I sometimes fidget finding that sweet spot for stance. Knee pads are a must. I am fairly athletic and young so it fits my motivated style of hunting. Saddle hunting is unmatched by any other elevated hunting method in terms of packability for an entire mobile hunting system, maneuverability, and flexibility for tree selection IMO. I can be set up and ready to hunt from any tree 6" up to 30" diam. or so, in about 6-8 minutes, and quietly!! The only trees (in my area) that suck to hunt from are shag bark hickory and cherry - the bark is crackly when tightening rope tethers/ foot holds- but they would be noisy for any type of treestand to be setup on the fly.
As far as safety - I've never felt more safe while hunting elevated position, than when hunting from my saddle. It serves as a saddle and harness in 1. I run 11mm Sterling super static 2 rope for my linemans rope and tree tether. The break strength is over 6K lbs. Plus the ropes are always in my sight and or going through my hands, therefore any damage would be detected. Can you say that every time you climb into your treestand you know the ratched straps etc arent compromised?? I know I check hang on stand straps before the season, loosen during off season if the stand stays up, but hardly remeber to check them during the season. This entire practice of leaving stands hung - which I would venture the majority of hunters do, leaves openings for strap failures to go unnoticed and ultimately accidents to happen.
Check out a new saddle that just hit the market. The Tethrd Mantis. Developed by saddle hunters with years of experience.
Also check out saddlehunter.com for the best saddle hunting forum with a plethora of information on the method.
https://tethrdnation.com/