The rules system is very simple regarding anything not involving magic, which is very good since it does not detract from the focus of the game. Checks mostly use a single d10 roll plus modifiers, the higher the better. There are no classes or levels, so characters are defined completely by their ability scores, skills, and advantages/disadvantages.
The magic system is relatively free form. Mages learn techniques (change, create, destroy, etc.) and forms (earth, body, magic, etc.), which are called by their Latin names. These allow you to attempt any kind of magical manipulation, although you need to learn formulaic spells to accomplish really powerful magic. This is very interesting to play and kept "balanced" mostly by the fact that all major PCs are wizards.
The style of play differs somewhat from D&D as the players are supposed to run a covenant of mages, including minor characters. So everybody creates multiple PCs, including mundane characters, and plays a different one in each story. This is a good thing, since real advances in magical power are only possible if you study for several seasons. So while the mages themselves may be busy in the lab, their underlings are sent on various errands. This also makes it easier to take turns being GM, which is the suggested mode for the game.
However, so far I have played in a more traditional adventuring campaign, with a group of mages searching for a place to set up a covenant. So I can't tell you much about how the above works out.
Overall, Ars Magica is a solid system, well suited for its purpose. My biggest gripe with the game would be that the rule book (4th edition by Atlas Games) is not well structured, IMO. I.e., it is somewhat hard to locate a specific rule quickly. Since we don't mind if your GM makes rulings "on the fly", this is not a big problem. Still, I sometimes feel that part of the rules is "dead weight", since we never use them.
