I have been waffling on this lens since it was announced. As well as the Sigma and Tamron comparative lenses. Initially I settled on the 300mm f4 PF, using the 1.4/1.7/2.0 TC's to get to the "longer" end, knowing that I was sacrificing the zoom. This past weekend I rented a 200-500 for a boat trip out of Neah Bay Wa to photograph Pelagic Seabirds. Our weather was not optimal for photography, foggy in the early morning, clouds and smoke in the air the rest of the day, the smoke from the fires in British Columbia. This gave me the opportunity to use the lens in tough conditions, which are not unusual here, versus the "good light" shots that we normally see. I have not processed too many images yet, overall I was not unhappy with the performance of the lens. To give you an idea of conditions, here are the 2 images I have put online so far, these are 2 from a Black footed Albatross landing, about a 20-25% crop. Out of a series of 10-12 images, I probably had 7 that were in really good focus. To be fair to the lens, the boat was bouncing, the seas were about 2-3 feet, and the bird wasn't still either. I was kneeling on the deck trying to be as steady as possible. 1. 2. I did these 2 first simply because the bow wake cracks me up. One other thing I really wanted was to get a good sense of the difference between using this and the 300 PF, which I had a chance to do. The good news, if I could get a substantial area of the bird within the entire batch of focus points, I was shooting d25, focus acquisition was generally OK. I did have the focus limiter on so the lens would not "hunt" through the same range, but when it started to hunt it was far more difficult to get it back versus the 300 PF with a 1.4 TC. I also found that tracking with this lens was not as accurate as with the 300 PF with the 1.4 TC. One time, interestingly enough with this Albatross, the zoom did help, but as others have noted it is tough to go the full zoom in one motion. In my case, going from about 250-400 was not difficult. My conclusion is that this is a super "good light" lens, but once the light isn't so good, the lens doesn't shine. For grins I tried mounting a 1.4TC, the images I have seen with this combo have quite good, again in good light. In these conditions fuhgeddaboudit. My recommendation is that if you want a "good light lens" at a very good price, this is not at all a bad choice. However if you can afford the added cost for the 300 PF plus TC's, you will get faster, in my opinion, AF overall, especially in not-ideal conditions. Over the next few days I'll add a few more pictures to illustrate, including some from the 300PF that I took on this trip.