After a huge amount of research, I decided to go for the Canon 100-400 mk ii. I could have bought it cheaper on the internet, but decided to stick with Calumet, because they are an approved Canon dealer and I could go in to my local store and pick it up. They also price matched another main Canon dealer, so the final price was not really a lot more expensive than the on-line price. I didn't have my camera body, so didn't manage to try the lens in the shop. When I got it home, I got ERR 01 (no communication) I tried it on another body and everything appeared to be OK, so I wasn't 100% sure if it was the lens or my body which was faulty. I called Calumet the following day and the first thing the sales rep said was, 'Don't worry, We will order you a new lens straight away'. True to their word, the lens arrived two days later. I went in to Calumet they checked the faulty lens on their body and same error. Calumet were absolutely fantastic in how they dealt with the problem. If I had bought the lens on-line, I dread to think the problems I would have had returning the lens, waiting for a replacement etc etc.