For this event, McDonald's worked with a pair of aerospace engineering companies -- the same guys, in fact, who helped spearhead Google's Project Ara modular phone. The STRAW uses a novel J-shape with five inlet ports: four on the shorter leg and a fifth located at the bottom bend. When properly dipped into a Chocolate Shamrock shake, the four ports on the short leg will pull both chocolate and mint shake in equal proportion from their respective layers. The port at the bottom is designed to suck up the last dredges from the bottom of the cup like a normal straw would once you're nearly done.
At least that's how it is supposed to work. It turns out that these Shamrock shakes are typically blended, not layered, which utterly negates the point of having all these extra holes. If you do manage to find a franchise that will layer your shake for you, you're going to need to position the STRAW very carefully between the two sections in order to properly line up the inlets -- not a task you really want to be attempting while driving or even walking. Plus, once the level of shake gets low enough, the exposed inlet ports will begin drawing air faster than the bottom hole can suck in shake, which makes drinking the last bits quite challenging.
Between the mechanical challenges that I encountered and the STRAW's sheer lack of availability, it strikes me as far more marketing tool than eating utensil. Granted, we weren't able to find a location that didn't blend their shakes -- even when specifically and explicitly ordered as layered -- so there may well be some consumptive value that this thing provides. But there's no need for you to go out of your way to find one of them (unless collecting McDonald's memorabilia is your thing) because you're going to spend far longer looking for it than you will waiting for the shake to melt slightly, then stirring vigorously and slurping.