This lens is truly a masterpiece. It's extremely sharp across the whole frame (something that is very rare for this type of zoom).
Lovely bokeh, colour rendition and low distortion and out performs all other 24-70's.
It doesn't come cheap, but if you're a professional and having that extra 10-15% performance for nearly double the price of competitors is worth it to you, then I highly recommend this lens. If you're a hobbyist, I recommend the cheaper Tamron as it does nearly as good a job for a lot less money.