I take the point that it is what it is, but surely the slow speed is a drawback, so much as a faster lens is a plus, this is also the most expensive 300mm f/5.6 by quite some margin, at this price point I'd have expected something faster, and as has been suggested it might have been an f/4 all the way.
Obviously you have a lot more images that we don't have access to, but I'd like to see more 'real world' shots before I make up my mind. How about some shots of outdoor sports, or winter wildlife? That is probably what this lens is going to be used more for.
The size of the lens is one of the big features, they can't make a 300mm f/4 lens that'll stand vertical in a backpack. That's a feature that will matter to SOME people.
I'll shoot soccer with it in the spring and I hope to get out to shoot winter owls sometime before the snow melts. I already know it'll perform great.