This is an excellent all around lens and one of the most popular as a general "walk-around" lens. As an L series, the quality is excellent and it is quite sharp. I have this lens, and on a FF body, it's extremely versatile - from wide to short tele, and the IS is very effective. probably on my camera as much as all my other lenses combined. Here are the downsides: first, on a crop body, the widest setting is not especially wide (but you have that covered with the Sigma). Second, there is a known ribbon cable issue as the lens ages on some copies, and the repair is $200-300. Third, even though the cost is coming down because the MKII version is now available, the price you're quoting for a new, not used, copy is too low, which tends to indicate it's a grey market lens. That may or may not matter to you, but it essentially means that Canon USA will not honor the warranty. You may be able to get warranty work if you mail it to its country of origin. If you want to be sure, ask for the S/N prior to purchase and check it with Canon support. In short, great lens, especially since you have wider focal lengths covered, but the price is suspiciously low since used copies of this lens are selling for $500-600.