I bought one of these gadgets, and it arrived today.
It's a sort of electronic synthesizer - the default fingering is similar to a saxophone, though you can also set it up emulate the fingering of other instruments.
You blow into the white mouthpiece to control the phrasing and volume like you do with a real sax and there is also a 'bite sensor' that allows you to 'bend' the notes a bit like you can with a saxophone reed. The separate box shown in the photo is the wireless pick-up unit that you hook up to an amplifier/speaker system.
You can set it up to sound a bit like a sax, but it can also sound like a trumpet, accordion, violin... whatever you want really.
The 5000 model has only just been released, but the previous 4000 model has been around for years and is rated highly by some good players.
So far, I've found it rather confusing to play - not as similar to a real sax as I'd hoped - but I expect I'll get used to it. One advantage is that you can plug in earphones to practice without disturbing the neighbours.