I either get the train or a private car, depending on a few factors.
If I am alone or with just one other person, train is best. The TGV from the airport to Disney takes just 9 minutes. However, it doesn't run all that frequently so it is worth checking the schedule to ensure this is in fact the quickest. I just did it in August but can't remember the exact cost- somewhere in the region of 20 euros.
If there are 3 or more in the group it is usually better value to get a private hire car, this is nice and easy, takes under an hour and means you go right to your hotel door.