I have a Canon 100-400. It's the second one I've owned, and I also use another one at my job. I sold my first one because I needed the money for some landscape gear, but I bought another this past November during the Canon refurbished lens sale. And this last copy is (or was) a noticeably sharper copy than the other two, although I've had good results from those too.
However, after only a month, the lens failed -- it would show the ER01 code at any aperture except wide open. After calling Canon and doing some troubleshooting, it was determined that the lens needed to go in for repair. It came back with the problem corrected, only now it's horribly soft wide open, and doesn't produce a "usable" image unless it's stopped down to f11. After speaking with Canon again, it's now on it's way back for inspection/testing and hopefully... repair.
In the meantime, I decided to pre-order a Tamron 150-600, and it arrived today. I took it out for a short time in the snowy weather today and took some songbird photos. I have a blind in my yard (geek!), and photographed in the same manner/surroundings that I use my 100-400. I didn't have a lot of light, and I set the ISO at 3200 (6D) and just tried the lens out. And the results were surprisingly good. The AF seems to function at least as well as the 100-400, the extra reach is great, the IS (VR) works very well, and the lens - although a bit heavier and longer than the 100-400 - was pretty manageable to use. I was able to get some decent images between 500-600mm -- the best were at f8, but the lens performed pretty well wide open too. I struggled a little with the zoom ring - I found it a little stiff. That will improve with practice.
I noticed in post that the Tamron images were a little "duller" (less contrast/color)than my 100-400, which was easy to overcome with some adjustments. Overall I'm very pleased with my first batch of images, and I'm looking forward to doing some more shooting this weekend.
As for my 100-400, I sure hope it comes back the way it was originally. I can see having both the Tamron and the Canon. We'll see...
Canon 6D | Sigma 35 1.4 | 40mm pancake | 16-35 f/4 | 24-105L | 100L 2/8 | Tamron 150-600 VC
Tamron 150-600 VC Images