It's good fun, but of course like any musical instrument may not suit everyone (e.g. milandro...). The octave key system does take some getting used to, but by having two buttons above and two below the "rest" position, you can go up and down by 3 octaves in either direction. It's actually a lot better than the WX11, which has two buttons down and three up, which are more widely spaced and only give you 2 octaves down and three up. Note that none of the WXn controllers has any sounds built-in - they're simply MIDI controllers.
As milandro says, you do really need the VL-70m to get the best out of it, and ideally with the Patchman sounds which I gather (I don't have them) are streets ahead of the stock Yamaha ones. These are available outside the USA, but it's a hefty $350 price tag - I wonder how many they sell?
Although the fingering is like a saxophone (or can be set to be - there's 3 sax modes and one flute), it's not really got enough keys to truly emulate a sax, and hence is probably of limited use for "silent" practice. That's apart from the very different feel, and the rattle of the keys and hiss of air through it!
The prices for the ones on eBay right now look a bit nuts to me, but that probably reflects scarcity - I was quite pleased to get my WX5 for £250 three years ago, and could have had a VL70-m for the same, at the time...
All that said, you'd probably not lose money if you bought wisely now, and then decided it wasn't for you after a year or two. But you're right - it probably won't catch on, especially as EU regulations have prevented its importation.
Cheers
Jonathan