@ebiggs1 wrote:"... using manual focus and probably 400 speed film, life has gotten a LOT easier!"
Yes it is. I use to use and try several different methods to push Tri X and Ektachrome to get a little more out of it. I had a 300mm f4 lens that I really loved, still have it. It was my goto for film lens to do tele work back then. I bought the ef 300mm f4L solely on that love. But, I truly love the new 300mil also. There is, must be, something in lens manufacturing that works out well for 300mm. It just works!
And, oh lord, I am saying it, the ef300mm f4L works wel with the 1.4x tele con, too. Gives you 420mm f5.6 with IS.
We're gonna remembrt you said that, Ernie! 
Ernie,
I am glad I started with film and manual focus because it really makes me appreciate what modern gear does for us.
The 300 does produce "magical" images with and without the 1.4X and it is a really manageable focal length. I think the EF 800 has the potential to do magic also but I am on the early part of the learning curve managing this focal length. I am missing too many capture opportunities with the 800, hopefully that will improve significantly with experience.
These were shot with the 1DX III and EF 800 f5.6 on a monopod this morning while I had another camera set up on a tripod for a scene that never materialized. Grackles can be annoying but they are also interesting birds.
Rodger
Those was from roughly 70 feet away and crops were ~20% of the total sensor area. With birds, even with an 800 mm lens severe cropping is usually needed.
I am still trying to get a feel for how well this lens accepts a 1.4X because weather conditions and lighting haven't made for many photo opportunities. This is with the EF 800 plus 1.4X at about 50 feet. I shot it at 1/320 with a monopod with the image stabilizer on and the IS definitely did its job. But a better test will be getting some better lighting and having it mounted on the big tripod.
Rodger
