I have several large K'Nex sets. I haven't done anything original with them yet (other than a brief experiment in dual steerable wheels), but one day I may try building a small mechanical digital computer with them. I prefer the old colour-coded colour scheme over the newer colours. Of all of the sets I have, my favourite is still the Big Ball Factory. It is models of that kind, and the amusement park alternates, that K'Nex is best at, in my opinion. I find K'Nex harder to build with than building blocks like Lego, and so spend less time on it.
I have a couple of sub-pages here. The first is some pictures of the Roller Coaster set in my spare bedroom. The second adds more amusement park models to that.
The official K'Nex home page is here.