When to (not) play Fable 3 in co-op mode?

I think it depends on the amount of realism that you want to achieve, and what type of partnership (just for kicks, business, or marriage) you want to enter into with the other person (and in the game).

If you want realism, then it helps if the second player has previously played the game - at least enough to get whatever item of clothing helps with the realism.

If you're married to your co-op partner and you want to marry someone in the game (to follow whatever story line you're interested in), then they'll get mad and want a divorce if you bring around your spouse (which in my case was my real spouse. That got weird).

From a business perspective it's important to note (at least as far as I've been able to tell) that if you purchase a property when the other player is out of the game, you'll have to sell, and re-buy it when they get in game if you are to share the property.

Another issue when screen sharing is that aiming gets weird. This is really important when you're on the Gnome quest. You'll have a lot greater challenge finding them with two people.

Other than that, I think you can accomplish every other aspect of the game when you have two people (even if they seem like a 3rd wheel at times ;)