K'nex has been around just long enough - 15 years - that it may qualify as a classic, but if you haven't taken a look at it lately, you might be in for some surprises: Micro K'nex and K'nex bricks, for example. Also a new logo: bolder and more angular against a black background - not so interesting in itself perhaps, but in a store with a variety of K'nex sets, it can help steer you to the ones with micro & bricks.
The micro K'nex are 60%-size versions of classic K'nex parts. Since their first announcement, I have been hoping for a pure-micro set - a "pocket K'nex." But no such luck: so far they are just appearing as supplemental parts in regular sets, with bottle-shaped adapter rods to connect to micro to regular. In exchange for the special features they add to the models that use them, there is some cost in robustness. Smaller connections just can't be as sturdy as regular K'nex: The wind kept blowing over my windmill during my Gasworks Park photo expedition, and if the tumble was very far, I found reassembly required and occasionally had to search through the grass for tiny missing parts.
The K'nex bricks are compatible, brickwise, with the usual suspects, which provides some interesting combination options. Unlike most similar products though, K'nex bricks have rounded edges which provide easy fingernail purchase for separating connected bricks. All bricks (that I have seen so far) have holes for 5mm peg connectors, for either micro or regular K'nex. Which is just shy of being compatible with the similar holes and plug connectors of Lego Technic (and some regular Lego). Sigh.
The set I got to try out micro & bricks came in a new case design that was sufficiently warped that most micro pieces could conceivably find their way out the gap. Worse, it takes no advantage of the new brick connection options to provide a buildable surface. Sigh again.
However, there were more pleasures than disappointments. With or without the new parts types, K'nex remains Good Block Play.