I shot my daughter’s basketball game with my new to me Canon 70-200 f/4 L. In a word - wow.
My first handful of pics were a bit soft. Then I realized I had the 6D on One Shot AF instead of Servo. Then looking at my pics in Lightroom I forgot by quickly rembered the first dozen or so would be soft.
Compared to my 28-135 f3.5-5.6 IS USM:
Very few pics were underexposed. We were in a better than average gym, but certainly not optimal lighting.
Razor sharp. And many pics were wide open at f/4, and many were at both ends of the zoom range. Not an in your face difference, but very clearly sharper.
Color and contrast are incredible. In 2 or 3 pics I’ve had to bump them up very slightly in post. The rest need nothing.
I had more misfocused shots than usual, but no more than 10 out of about 325. I had the minimum focus distance switch set to 1.8m instead of 3m so I think that’s what did it; I think the focus is supposed to be faster at the 3m setting.
Once I got to about the picture where I remember I changed the AF setting, looking in Lightroom all I could think was “this is exactly why I bought this lens.” Every single thing is better. The only editing I’ve had to do is cropping and angle adjustment, which are on me and not the lens. Even the auto white balance setting was far more accurate. For reference, I’ve shot in this gym 3 times now so it’s not a case of the lighting was better; I’m comparing pics from those 3 times.
I just have to get used to the minimum focusing distance. I was front row under the basket, a bit too close when they were on the baseline. The pics at that distance weren’t soft, but they were slightly softer than the rest (comparing 70mm shots that were very close vs a few feet further).
I’ve read so many people say this lens is a gateway drug. Their first L lens and it made them replace the non-Ls in their collection with L lenses. I totally get that now. Maybe I’ll sell all my other lenses and get a 50 1.2 L?

Edit: I’m definitely going to keep my 40D for outdoor use for the 1.6x crop factor though. 200mm probably isn’t long enough for soccer.