Let's make sure we keep in mind the price/performance ratio on this under-rated lens. At the price most people end up paying for this Canon upper level consumer zoom the value is incredible.
That is to say, if you buy it as part of a camera kit with your 40D/50D/7D you're paying only $200 for it. If you buy it used from someone selling it from a kit you'll pay between $275-$300. At those prices, this lens is a bargain. The autofocus is fast and quiet. The zoom range, although a little narrow on the short end makes up for it on the 135 end. Image quality is very good, although it does suffer from CA it is not a deal breaker.
If you need a walk around lens that won't break the bank and will give you very good images, you know, the kind that DON'T look like they came from you kid's point&shoot, buy this lens USED.