After having this lens for 2 months now, I can definitely say it's by far my best and mostly used.
From Macro to its most zoomed, this lens is amazing. The clarity in my shots has increased ten fold, I would even say at certain zooms/apeture it competes with my 50mm prime. It's very quiet, is extremely quick with its focus too.
It takes great shots in low light too, and with my new Canon 7d, I can really push the ISO up and this lens is superb.
One minor problem with it, depending on your strength etc, is the weight. If you are intending on using it all day every day for a week, you will feel it. I recently took it to New York for a week, and after 7 days, I couldn't take it out anymore because my shoulder was really hurting. I personally think its because I had it in a slingshot bag, which only uses one shoulder (which is why I am changing to a back pack soon) but if you are less than average build then you will notice the weight.
In all, the price is high, but the clarity in my shots has increased more than the price from a normal lens.
If you have the money, go out and buy it. It's worth the purchase.