I have lots to say about Gear that I can't do right now since I'm at work (I'm trying to write an article on it in my "spare time", which I have none of), but I'll say a few things now.
I think Gear is actually the best card in Adventures. Not Page, not Magpie, not Hireling, but Gear. "Best" in that adding Gear to a deck is very rarely the wrong move, and if it is, it's because you wasted terminal space on cards that aren't Gear.
Gear in engines is actually quite a bit like Gear BM: it has a similar net effect of speeding up development. You can pass cards you don't need to next turn, make Victory cards miss shuffles / line up with trashers, line up excess money with +Buy, etc. Dominion is fundamentally a game of (among other things) probability management ("you make your own shuffle luck"), and Gear manages those probabilities better than most things.
Gear is of course best with cards that need to collide. Gear / Tournament is incredible, and things like Gear / Baron or Gear / Tunnel / Enabler or Gear / Market Square / Trasher are all pretty good.