I finally got one. A good deal on a used EF 200mm f/2.8 II came up, so I jumped it. The only telephotos I've used before this have been the Canon EF 75-300mm f/4-5.6 USM III and the Sigma equivalent. It's a pretty stark difference in quality.
My initial impressions from playing around with it on the 60D - she's a beautiful but demanding mistress. The viewfinder is much brighter now. AF is incredibly quick and and solid - no hunting at all. It's as sharp as one would expect for an L series telephoto.
But with great power comes great responsibility. I can already tell to get the performance out of this lens will demand perfection from my technique. Drop the shutter speed too low, to much shake in the hands and bam - the magic is gone. Focus is critical - even at f/32, the DOF scale is tiny! But at least I know when I perform, the lens will too.
I picked this up lens as it was high time to upgrade my telephoto. I'd been humming and hawing between a used 70-200 f/2.8, 70-200 f/4 IS when this one came up. I went with this over the other two because:
1. I'm an over-smug hipster who loves primes and sneering down on those lowly zooms. Okay, I'm definitely not a hipster but I do love primes.
2. Most of my use of the 75-300 was at one end or the other. Getting this I can still pick up an 85 f/1.8 for the other end.
3. Every review I could find of this lens sang its praises. The only real knocks I heard against it were it doesn't have IS and it's not a zoom.
4. I couldn't get everything I wanted from either zoom: I wanted this lens for both hiking and sports, so I wanted the 2.8 aperture but also a light weight lens. I'd like IS too, but that's more like icing on the cake for me.
5. It's black.
6. Reliability. The 70-200 2.8's are (according to lensrentals) more likely to require repairs over their life. It's the nature of the beast - they're moving a lot of heavy glass around. I wanted to minimize the chance of the lens needing a visit to the shop, especially since I was buying used. My 50 f/1.4, currently on its second trip to Mississauga, has only reinforced this
Those were the main things that guided my decision, naturally the thinking will be different for everyone. I'm excited to get out with it!