9 out of 10 points and recommendedCompact, lightweight, solid...great imagesPricey for f/4. Wide end requires post-pro.
I purchased this lens after I a dicey night photographing a concert with a non-canon f/2.8 lens, whose AF couldn't keep up. I noticed, however, that my 70-200 f/4L had a much higher hit rate, and more keepers despite the loss of one stop. So I took the plunge on the 24-105 f/4L and what a difference! Even in low light, the AF is fast and sure, especially when I switch to AI servo and turn on AF expansion. This lens is used on a full frame 5D.reviewed March 1st, 2008 (purchased for $800)
To summarize: excellent low-light performance, and exceptional camera-to-lens interface.
Optically, this lens produces sharp and contrasty images, and the IS performs extremely well. I feel that it does exceptionally well at capturing people and have had a rewarding experience with events photography (weddings, concerts, some coprorate work). The bokeh produced by it's 8-blade diapragm I find to be extremely pleasing.
At the wide end, there is vignetting and (as one would expect) some barrel distortion, and these require post-processing. Canon's DPP v3.2 and up has built in lens correction algorithms, so this is a one-click fix. Other software tools also will make short work of this (Photoshop, PT Tools, DXO).
My only gripe is that this lens is offered at nosebleed f/2.8 pricing levels, otherwise I would have given this a perfect score. I bought mine for less as a fully-warranted Canon factory refurb from my pro shop - I probably would not have paid full fare for f/4 at current msrp.
But you needn't worry about its performance in low light. Other than a slightly darker viewfinder, it's a superb low light performer. And a future camera body with a live view OLED screen (and/or EVF) will moot the viewfinder issue.