As a poster above said, what if the IS or AF motor does go out on the Mark I? How much pain in the butt would it be to get it fixed by Canon? Either the AF or IS? I've never had that happen in a lens, so I was just curious.Cant answer that question ... but want to say thats a good reason to buy the non-IS version if you can find one. Ideally, the newest version of the 400 f/2.8 is best, ... if you can afford it. But when buying only what one can afford and it being older, I think it's best to eliminate the 3-$4000 total loss of a failed IS motor.