A number of years ago I purchased my first 70-200. It was the f/4 non-IS version and I thoroughly enjoyed that lens. I soon felt the need for a faster aperture and purchased the f/2.8 non-IS version with which I was very happy. Image quality was very good and the weight was manageable. However, not long after, I decided to upgrade to the IS version of that lens but was never totally happy with the image quality it delivered, especially at 200mm and wide open.
When the 70-200 f/4L IS was introduced and got rave reviews, I decided to give up the fast aperture and opt for the improved image quality. That turned out to be a fantastic lens and I loved the images it produced. Then along came the new f/2.8L Version II and because of the reviews I couldn't resist. I coughed up the money and bought one.
That was one of my best ever purchases. In a word, this lens is spectacular in every way. The AF is super fast and accurate and the image quality it produces is simply outstanding. At any focal length and any aperture. I am in love with this lens. I believe it is as sharp as my 300 f/2.8L IS and my 500 f/4L IS. Yes, you read that correctly.
I shoot a lot of action photography, including race cars, superbike races, motocross races and dog events. This lens goes along with me everywhere, right beside my 300/2.8L IS. It's indispensable.
Was it worth the money and the extra weight. Without a doubt. I would buy it again in a heartbeat. If you are looking for a 70-200 and have the funds to buy this one, GO FOR IT. You won't be sorry.