I bought this lens a week ago, while on a recent visit to LA.
Initially, I had a choice of purchasing this lens, or the Canon 24-105 f4 IS L, however, requiring the f2.8 meant that this lens ended up in my camera bag!
This is my 3rd L lens (already own a 17-40mm L f4 and a 70-200mm L f4) and although I was used to L-lens image quality, this lens blew me away. Very impressive. The f2.8 yielded fantastic portraits.
Lugging this lens around (attached to a Canon 350D) through various theme parks and tourist attractions eventually resulted in neck and shoulder pain. The gigantic hood also means you attract a lot of attention from passers-by. However, this was a small price to pay for the amazing shots this lens produced.
Comes highly recommended if your budget can allow it!