The 24-105, IMO, is one of those lenses that gets a bad rep just because so much stuff is repeated second hand on the internet. The 'distortion' everyone talks about is there, but in most cases hardly noticeable and even then easily correctable with the excellent lens profiles availble in DPP and the Adobe products. 'Sharpness' is subjective, as what is sharp enough for one may not be for another. Pixel peepers are often disappointed with gear that others are making excellent large quality prints with, so I tend to consider that fact when discussing sharpness.
I 'upgraded' to the 24-105 recenty from a 28-70 F2.8. My old lens is in need of repair and I'm looking for a shop that still has parts. In the mean time, i did not want to pay the current prices for a 24-70 F2.8 because they were in high demand and expensive even used. The V II is way out of my range.
I thought at the time i would be 'settling' for the slower lens, but it only took about 5 minutes after I put it on the camera to ask myself how I lived without the 24-105 for so long! It is lighter and more versatile than the 2.8 lenses in the standard zoom range, and i find the IQ to be very similar. With the newer bodies having higher ISO performance, I hardly if ever miss the extra stop of light. Though, of course you can't recreate the F2.8 depth of field with an F4 lens at any ISO
In summary, the 24-105 is an excellent lens, particularly considering is versatility and weight.