I've finally got my hands on an Art-series lens.. after seeing some of the lenses in this series beat out the more expensive Canon and Nikon-brand equivalents I was pretty confidant that I wouldn't be disappointed with the 24-105mm f/4.
It focuses very fast between near and far objects and produces nice sharp images.
I'm not sure whether it's a downside or not but the first thing I noticed was how heavy this lens is.
It's almost a kilogram (885g)... it's great how it makes you feel like you are holding a quality piece of equipment BUT you do start to feel it after a while! It also makes your filter purchases a bit more expensive as it needs 82mm filters but these are very minor negative aspects.
It's not as fast as its brother, the 24-70mm f2.8, but it's also half the price and the focal length range makes it a very versatile lens.