
I dont think there has been one bad review. I think there are a lot of very high expectations to get over. Every review appears to gush about the IQ and the build. I like mine so far. The price? Not so much. I am still trying to justify that and if I cant then I wont keep it.....
The biggest problem is copy variation. It's not very promising to see big reviewers mention that they had X number of copies that had to be sent back. When someone pays $2,300 for a lens, they expect it to be perfect. Not everyone has the tools and knowledge to test a lens inside out like they do, so they will always have a "what if" worry in their head. I honestly will never buy this lens unless these issues are fully ironed out.
I'd like to remind again that 70-200 IS II was a similar lens in terms of the price point, but it has had none of these issues and every single review of it (and everyone else in every forum) praised it from day one. I think people were expecting to see that which didn't happen. Since it happened with another lens (and another Canon lens), it's not unreasonable to expect the same outcome.