I'm not a completely versed user yet but the red AF illuminate (Auto or ON) only seems to consistantly work for me in One Shot mode. If I have the camera set to Ai Servo and a zone tracking type AF (still adjusting from my 5D2/40D days) the red AF illuminator doesn't seem to always activate, or blink, or whatever you wish to describe this as. If I tap the [-|-] AF button I can illuminate all the focus points and the active point however.
I can certainly see where some folks take issue; that said, I guess I'm not as bent out of shape - I certainly find the AF and shutter response on a whole worlds better than my 5D2 regardless. So I'll take this over the previous, but this seems like something a firmware may be able to adjust or tweak from an illumination stand point. But I'm far from certain.
I'm certainly not going to call this camera flawed and unusable though, I really do love this update. I'm thrilled to have this AF setup, having only ever used the Canon 7 point Elan 7e and 9 point 20D style systems for at least a decade, I find this system great.
But I can agree with others in very low dim light (lower than I can ever remember using my 5D2) the AF points are difficult to see in Ai Servo mode. However, I'm not certain my 5D2 could even work in light as low as I'm willing to push the 5D3 now, so again, I won't go too hard on the camera just yet. And frankly I rarely used the 5D2 in Ai Servo because in many cases I didn't find it snappy enough to warrant the use, so using Ai Servo on the Mark III is a new experience for me after 3 years of using the Mark II.