I've not played or even seen it, but Tzolk'in really intrigues me. I read the previews earlier this year, and I promised myself I'd keep an eye on this game.
Well, I lied to myself and forgot about it until recently. But, looking at the BGG page, it still seems to have a lot of buzz. I suspect the gears are more of a gimmick. I read someone deny that it's a gimmick, but I'm sure one could do the same thing with different tracking grids. And if a gear should break, I'm sure that's exactly what you'd have to do.
But it looks like a decent game. Now that it's come back to my memory, I'll have to read up on it some more and see if I want to purchase it.
Tzolk'in is really good, but it's definitely best with 4, not with 2 (I've played both, but not 3 player). Strictly speaking, the gears aren't needed - you could do everything with tracks, but here's the issue:
With tracks you have to:
Advance every pawn exactly one space, on each of five tracks
Advance the turn track exactly one space
Be very careful not to make mistakes as you perform this bureaucratic step.
With gears you have to:
Turn the gear one tooth.
So yeah the gears themselves aren't strictly *needed* but they're definitely the best way of implementing the games system (at least, that I can think of!)
As for the game itself, it's really good. I've played three very different strategies over three games, and had a lot of fun each time, and not felt anything was too overpowered. It's a game that blends strategy and tactics really nicely - you play with maybe a goal you're working towards e.g. get this wonder which gives me 4VPs per green building and build more green buildings, but each turn you have non-trivial decisions, as every turn your workers automatically advance to (typically) stronger spaces, but you can't do nothing, so do you take these two good actions now, or split them over two turns (bearing in mind you can only place OR remove workers on a turn, not both)? And of course there's lots of ways to do things - do you build more green buildings by using the building/tech gear and pay in resources, or do you go for the high value trading space that lets you pay in (comparatively less) corn? Maybe you should try and get to the building gear's double build space, but ah, then you need to do something else with your other pawns, and food day is coming up.
Well, man, I don't know exactly what writing style I was going for in this post, but if someone you know ever gets Tzolk'in, definitely try and get a game of it in. I'm pretty indifferent to worker placement, but Tzolk'in is awesome.