Last week I recommended Microscope on the basis of downloading and reading it. Today I can recommend it on the basis of playing it. A friend and I sat down for a couple of hours and gave it a run through, and enjoyed the experience.
A caveat: there were only two of us, and though the game works with two, you really need more than that for it to run properly. But in spite of that I felt that it came together surprisingly well: initial awkwardness about what to do during a "scene" - which is basically like free-form role-playing within a certain structure - quickly faded and we got into the swing of things very easily. The history we created charts the life time of a city, from founding to dissolution, and encapsulates a Mantis God, ab-humans, anti-daylight, Big Fuck-off Pterodactyls (BFPs), and street wars between merchant guilds. We filled out bits of it in detail, skirted over others, and left huge swathes unexplored.
I wouldn't call Microscope a role playing game. I'm not even really sure I would call it a game. (Not that it particularly matters.) It's more a set of tools for structured brainstorming: a means by which you focus your creativity like a lens on different subjects at a time and make them grow. And it is quite potent stuff. Properly guided, a group of people can come up all kinds of crazy and wonderful nonsense within a surprisingly short space of time.
This is why I think the potential criticism of Microscope - that you might as well just sit round as a group and create whatever your want; why bother buying the game? - misses the point: structure, to a certain degree, is important. It stops you meandering, drowning in a sea of too much choice and freedom. Microscope gives you just the right amount of discipline, providing a framework in which group creativity can flourish.