Gear Cube looks a lot like a Rubik's cube, with its 3x3x3 configuration of pieces, however the pieces have gear teeth on them which makes for quite an unusual motion. Due to the gearing, you can't rotate each layer independently. Instead, the middle layer turns one quarter of a turn when you turn a face by a half turn. This means that you can't actually do a quarter turn since that would cause the middle layer to only move one-eighth of a turn, which blocks other movement.When you rotate a face, the edge pieces of the middle layer rotate as well, causing all sorts of apparent chaos.
This is probably pretty difficult to understand by reading it, so check out this video of the cube in motion by larfrtc:
As you can see from the video, it has a nice smooth motion. The edges of the gears are rounded a bit, and it has a fairly loose tension to avoid pinching your fingers like in the original 3D printed prototype, dubbed the Caution Cube, as Oskar explains in this rather humorous video:
With all that is going on with this puzzle, you would think that it would be pretty tricky, however it actually isn't too bad. I'm not all that great at twisty puzzles, but I was able to scramble and solve it in about 15-20 minutes. That was more just blundering towards the solution, however, but with a bit of practice and thought I was able to figure out how to reliably solve it.
I really like twisty puzzles that I can actually solve, so that's a big plus for me. That said, it isn't trivial to solve either, it will take a bit of thinking. Also, it just looks really cool: this is one that I really like to show to guests, though none have been patient enough to try solving it yet. Overall, Gear Cube is very nice puzzle!
There is another version of this puzzle called the Gear Cube Extreme in which four of the edge pieces do not have gears, which makes it a much harder puzzle once it gets scrambled up. I probably won't be trying this one anytime soon, but check it out if you're looking for something more challenging!