The 300 2.8 mkii, and the combination with the mkii extenders, is a marvellous piece of glass. I mainly use it for sports, not inside as the 300mm is mostly to tight for there. However, on a racetrack you will not find a better if you want to pan. The af is very fast, even with the 1,4 added. So, within a few weeks I will be in Francorchamps for the F1.
I'm not a birder, but last Sunday a couple of sperwer hawks came flying on. I was too late as I needed to fetch the 300 inside, I really missed a nice opportunity there. I got some shots of them with the 2x on the 300mm, but I have to crop that much that the photo can't be used.
Also wildlife (I will visit South Africa in October ) is begging for such a lens, with and without the extenders and is also travel friendly.
I also took portraits with that lens and they are really good.
In September there will be an aviation show close at home on the military airfield of Kleine Brogel.
Most remarkable is the isolation of the subject that is created. I mostly use this lens on 2.8, even with the 1,4 it stays on 4. For adding the 2x I mostly choose for f8. But be aware in that case you have a 600 in your hands. I rarely do use this lens with a tripod or monopod. This out of hand shooting works quite well after some practice.
Satisfied of this Lens? Yes, it's a real big tool. Never expected that I would kill my 70-200 2.8 mkii in such a way. I know it is not cheap, but worth every penny. Prices seem to drop now to 6150€, coming from 7000€ 6 months ago in Belgium. I did afma this lens and extender combinations on the 1dx, 5d3 and 7d with Reikan Focal. Without the results were not that good. On the 1dx and the 5d3 this lens is doing everything you could ask for. On the 7D the results are not that good but that's caused by the af of that body. However it offers almost a 500/2.8 on that 7D and that's great. So I hope the new 7D2 will have a better AF and then this 300 will match perfectly.
One remark: GAS is also my problem. I will buy a 200-400 1,4 soon. I think that even then I will keep this 300.