Ars Magica, first 3rd edition and then 4th edition, was the system in which I participated in one of the longest running campaigns I've ever been a part of. Our GM did an excellent job at making us feel frustrated. In the end, I think the other guys managed to prevent Harmageddon - I'm not sure. The GM didn't tell them straight out . That's typical of him, by the way, but that's another story entirely.
RL matters forced me to leave the campaign - and the city - before it was concluded. That's still something of a sore point with me. We had played something like a year, year and a half, and the campaign still ran about half a year before it concluded. I really would have liked to see it through to the end, but that was unfortunately not possible.
I did, however, create a few files for Ars Magica - mostly writeups on grogs and whatnots.
What is interesting with Ars Magica, is the concept of the 'troupe'. Obviously, there's a large group of characters to choose from when one starts a game - each character usually creates one mage, one companion, and two grogs - with grogs being the 'common folk', companions being 'exceptional beings', and mages being, well, mages. However, there's no set ownership on the characters. Obviously, the mages usually belong to the character that created them, but otherwise the player characters for that evening are chosen among the likely candidates.
Thus, a number of interesting options arise. Let's say that one mage decides to investigate some strange occurrences in the vicinity of the Covenant (which is the home base of sorts for most, if not all of the characters). None of the other mages are interested in tagging along - they're not interested in such occurrences, there's a very nice lunar equinox coming up, it's Saturday for chrissakes, and whatnots. The GM decides to run a session where the mage investigates those occurrences. However, the mage obviously takes a few people with him - it's just downright foolish for a mage to run around completely unprotected. Thus, the others choose, say, a grog each and one chooses a companion - or the GM merely allocates characters, based on their descriptions and who is available.
It may sound complicated, but that's just because I tend to explain things in a complicated way. It played very nicely, actually. I like Ars Magica .