I have the 17-55 2.8 like you and i have pondered the same question, mainly for portrait and wedding shoots. I have seen one in Jacobs, the build quality is nothing near, i love the wide open aspect of the 2.8, but could use the extra reach, but i have not made the jump yet, i keep hoping someone in my local camera club will buy it and i can have a go with it. A couple of people on here have had the 24-120 f4 and sold them on, so that is another reason i have hung back. I keep thinking save up and get the 70-200. So if you buy one i would be interested how you got on.