the autofocus is slower
How is the auto focus?
This is a fantastic lens considering its price, its zoom range, its compact size, and its light weight. It's by far the cheapest lens I own (the others are the $700+ Canon 15-85, the $400+ Canon 60 macro, the $400 Tamron 28-75).I considered getting the Canon 70-200mm f/4 IS L lens but for 5x the price you get less zoom range, a heavier and bulkier lens, and perhaps a little more snob appeal with the white barrel and that precious red ring.  No doubt the L lens surpasses this one when you photograph test charts (how often do you do that?), and its weather sealing allows it to withstand thunderstorms better, but frankly speaking, if you're not a pro who pushes the envelope, who cares?If you have a crop camera, this lens lets you take advantage of the smaller sensor - you're not paying for (and lugging around) all that extra glass.  Maybe the extra $800 you spend on the L lens will make a difference in 1% of your photos, but you realize the benefit of the lighter weight and smaller size 100% of the time.  Plus you're more likely to bring it with you.Yes, at longer focal lengths you lose one f-stop compared to the L lens.  But at those focal lengths, DOF is short anyway, so you can still get portraits with blurred backgrounds at f/5.6.Sure, the front element rotates and the autofocus is slower, and you can hear the IS at work, but are those worth $800? 