I too have the 5D MKII. In good light, it is accurate. It even tracks well. I have taken pictures of dogs at dog parks... lots of them. I hardly need to toss out for lack of focus and this is on the 70-200 f4 non IS> It tracks even when the dogs are moving towards me! However, they are not reliable in low light. Only the center point can be trusted for that. It does not mean that you cant get a shot in low light with the outer points. It just means that you do start getting out of focus shots at a near 50% rate or higher.