Just wanted to add that I have had the same experience (with 6D). I can understand if a lens is not as sharp as the primes, but this lens is considerably soft and I found a lot of threads talking about this softness. Another comparison is here.
Over the weekend I was out shooting with both the 24-105 and the 70-300 4-5.6 IS USM lens. To my disappointment, the L lens did not perform any better than the cheaper zoom lens. I know some people say not to pixel peep, but there is a functional reason for that, such as when you're shooting wildlife and need to crop. Furthermore, if an expensive lens is going to perform at the same level as a much cheaper one, financially it makes little sense to own it.
At first I thought I might have a bad copy (MFA is ruled out), but looking at various scientific lens reviews and also seeing many threads like this one, it's clear that the lens was just designed this way. This is unfortunate, because there are many other lenses that perform at the similar level, but are half the price and have much larger reach and/or speed. It's likely that this lens would be viewed as another ordinary lens had it not be for the red ring and $1k price tag.