I bought a saxholder yesterday, and thought I'd post my impressions here, rather than starting a new thread.
First of all, it WORKS. And I mean man it works. It makes a tenor feel lighter than an alto. I usually use a Neotech super harness, and the saxholder makes the sax feel lighter than with the harness. I practiced for some 4 hours today with the saxholder, and felt much better and was more productive than usual.
The drawbacks, however, are significant. Firstly, it feels relatively cheap. I could easily snap it in two (the front bar part is all plastic, including the swiveling abdominal plate. That said, to be stronger it would have to be made of metal, and then rubber-coated, significantly increasing the weight.
Secondly, if you play with the sax on the side, things get weird. Straight in front of you, it's great. Move the sax to the right a bit, and it gets weird. Move it fully to your right side, and it's game over. It's hard to describe what's going on, but basically the whole apparatus will lean onto it's right side, so the right side of the ab plate is going into your belly (rather than flat onto it), and the right shoulder hook cuts into your neck as the left shoulder hook lifts up. So playing sitting down isn't all it should be. I think a sling is the best option for sitting with a tenor or bari.
Third, there were some strange design choices. The shoulder hooks are bendable metal (perfect), covered in rubber treads (***!!!). There's a reason why all other straps use things like neoprene, leather, or memory foam-- because they're GOOD. I also think the hooks should be flat metal, rather than tubes (see snare and bass drum harnesses for what would work well).
Also, the ab plate is about 3 times smaller than it should be, the wrong shape (flat), and the wrong material (plastic). It also doesn't go down as far as I would like. Additionally, it only pivots on one axis. It should really be a ball joint so the plate would stay in place when you move the sax to the side (rather than tilting on edge, what it currently does).
Finally, if I stand up straight with my shoulders back and chest up (like I should ;-) ), then the plastic bar in front pushes against my sternum. So there's pressure on my sternum as well as on the abs. This last point may just be due to my build, though I am quite average. I think the front bar should arc out.
If I keep it, I'm going to glue a gel wrist pad (the type that goes under your wrist when using a mouse on a computer, about 4 inches by 5 inches by 1 inch) onto the ab plate. I have one taped there now, and it's much better.
I will then cut some thick neoprene like is on the neotech harness and make sleeves to go over the shoulder hooks. I put on the neotech harness and then the saxholder on top of it and the saxholder becomes WAY more comfortable (shoulder hooks over the neoprene).
I should mention that within about one minute of testing the saxholder in the store, the rubber parts on the shoulder hooks pulled threads out of a brand new shirt I was wearing (higher-end polo). DON'T wear the saxholder directly on a shirt you care about.
I'm still trying to work out the physics/ergonomics to see if it's possible to make it stay relatively in place while the sax is on the right side, but I'm guessing that if the designers couldn't do it, then I can't either.
My recommendation to the curious is to wait for a version 2. If you have back pain and play bari or bass, then maybe give it a shot. If you play alto or tenor, probably try a good quality yoke strap, harness, and/or sling first. If you're a rocker, move around a lot, swing the sax to the side, and bend back and lift the sax up (and that kind of stuff), try a sling. If you have an opportunity to try out the saxholder before buying, definitely do so; it will trip you out!
EDIT: I want to mention that I have a yani tenor. The default cord on the saxholder is supposed to be the right length for tenor (I think), but I had to shorten it quite a bit. Maybe the hook hold on yanis are located at a different place than other saxes. Also, presumably using the saxholder with bari or bass would have the cord longer, so you may have better mobility/swing/side-playing with those than tenor. I don't have a bari to try it on.