I have not had a chance to follow-up on the T1i and 5D3 comparisons from a while back, but ...
I did shot again this weekend with 5D3 and 600EX at night and it was an exercise in frustration to say the least. Here is what I observed:
1. As noted by someone else previously, distance plays a big roll in the AF Assist beam effectiveness. I was shooting outside in a town square, very low light, 100mm L, trying to get my wife and son shoulder up with a bit a of Xmas tree lights in the background. At the distance necessary to make that framing possible the focus assist was virtually useless, not locking at all or shooting completely OOF shots. This was not remedied by the suggestion of not waiting for focus confirm and just pressing the shutter release button all the way (it would get me OOF shots when it would actually fire). Turning the flash off resulted in a focused pictures (not instantly, but acceptable given the low light). I did not have my T1i handy to compare but I did not have this problem shooting the same shots in the same square, at the same time of night, with the 430EX II and the T1i. (Not scientific, I know but there it is...)
2. At closer distances, the focus assist worked but slowly. I would get the shots in focus but I missed the moment because of the hesitation. This was maddening to say the least. Here I am with $5K worth of camera kit and I could not get a decent shot that would have been possible with a lot less.
I continue to be disheartened and hope that the issue is identified, a fix is possible, and that it will be release within the next few months. I'm kind of gutted right now.