I have known about the existence of K'nex for quite a while, but since I have been spending a lot of time hacking Meccano, the last thing I needed (I thought) was a new construction medium. What changed my mind was two things: I noticed some K'nex sets heavily discounted in a local toy shop. I then came upon Mark Nahabedian's Knex Page that included a design of a Digi-Comp I in K'nex. I remembered this toy from my youth and decided to try it.
Here is a page about my build of Mark's design.
The first model that I have designed in K'nex is my Think-a-Dot. Here is a page about it.