Hi Mike,
Have you had a look at the POTN website? (
http://photography-on-the.net/forum/index.php) This site specialises in Canon Gear, and has a whole section devoted to lenses, both Canon and Sigma, Tokina etc etc. You would almost certainly get some opinions from there. The 24-105 L IS USM looks good, but you would want a healthy bank balance to buy it. ($1600 -1700 +GST)
Regards,
Mike