I don't care that I can't actually touch-type all those keys; as long as all the mathematical keys, Greek letters and European accents are somewhere on my desk, that's good enough. I want to be able to press it, not fiddle around in software to insert my characters.
I think it'd be best as an extra keyboard just with these characters, rather than a replacement for QWERTY. Is it possible to have 2 keyboards like this? I'd definitely buy one