Opinion: I brought this lens a couple of months ago and have been out taking photo's with it at every opportunity. I have now taken about 10,000 photos with this lens on both a 60D and a 5D mkII mostly of Birds and It is just amazing and worth every dollar spent. I am not a professional photography but I am working towards at least becoming semi professional.
The lens works very well in the mostly outdoor settings that I do the majority of my Photography and even with a 2x extender it produces some really jaw dropping images. I have a backpack that can take this lens and three other L series lenses plus two cameras, laptop, flash, extenders, and bits and bobs and do not find it any problem to hike over 10-15 km over a day.
If you ever have the opportunity to own or use one then I would recommend it.
Problems: Not much of an issue once you get the hang of it but when using a 2x extender it helps to manually focus out to infinity and let the AF come back to the image as it can take a while to find and focus on low contrast or distant subjects.