I ordered the 24-105L with the intention to replace my 17-85 IS. I was very excited when the new lens arrived but after shooting with the lens both indoors and outdoors I was very disappointed with the IQ. I could not tell much difference between this one and the 17-85. The resolution, colors and contrast are very similiar.
It seems to be either a hit or miss with this lens, and with the high price tag that's just plain wrong! I thought of exchanging it with a new one, but trying to fish for a "good copy" is just too much hassle so I sent it back. Happy to save my money!