
I am probably alone in my thinking, but the 24-105mm f/4 should NOT be marketed as an "L" class lens.
For its focal range, it is too slow to be anything special, and the secondary market used prices, show its TRUE value among photographers.
Is the focal range useful for everyday shooting?... Yes, of course, but there are many lenses with useful focal ranges.
Does the IS make it sharper than otherwise normal?... Yes... but many kit lenses have IS.
In my humble opinion, the 24-105 is BELOW "L" level expected quality, and above cheap kit lens quality.
You are better off buying the 5dmkiii body only, and the new Tamron 24-70 f/2.8 + Stabilization.
Then, in 2014, get a Sigma 24-70 f2.0 + OS.
It has L built and weather sealing, silent, fast and accurate USM focusing, constant aperture. what does f4 have to do with it? canon has a trio of f4 L zoom lenses. and two L tele zooms that I can think of which are f4/4.5-f 5.6.
I have gone through 4 copies of tamron 24-70 VR lens, and have done exhaustive testing on all of them, and none of them have the even performance of 24-105. all have had some degree of decentering issue, some terrible, some mild. I would use the 24-105 happily over the tamron, anytime f4 is fast enough.
As for the price of the 24-105, it sold extremely well when they were charging 1000 bucks for it, and a few years ago, I sold a used copy for 850, which is about the usualy drop in price in used equipment. the reason that used prices have dropped so much is entirely b/c Canon is giving them away so cheaply as a bundle. Why? price pressure from Nikon. the deal of 6d with 24-105 and printer for 2000 after rebate from adorama was a smoking deal that I go on. the lens has been an ace in canon's corner, and a reason that many people on the fence between the d600 and 6D kit have chosen the 6D and 24-105. It also makes 5d3 a lot less absurd in its pricing. Of course that has led to a flood of market with these lenses, as many people who have this lens already, are buying the kit, to end up with a cheaper camera, and of course those that no interest in having this lens. That is what has driven the price down, law of supply and demand.