Does usefulness - for want of a better word - come into this? People keep coming up with stuff all the time, but plenty of it is "out of its time" by not having any practical application.

*nods* Definitely. Look at Ada Lovelace: she came up with a bunch of computer programs that would have been really useful if there were any computers to run them on :D

If a place for such an invention shows up later on then the inventor is thought of as being ahead of his or her time, but what if it doesn't? Would such a person be thought of as a lone genius or just a mad inventor?

Good question. Hmm.