Which one is it - the 85 mm 1.4?
I already had a 40mm f/2.8 STM "Pancake" , 50mm f/1.4 , 85mm f/1.8 and now I have the 24-105mm f/4 L (IS USM).
The 24-105 L is a beast, built like a brick out-house. I bought it because all those Wedding photographers, whose livelihood depends on such things, can't be wrong.
F4 on a full-frame gives a nice shallow depth of field once you get to portrait focal lengths. Having the super-shallow DoF on the prime lenses is nice but best used sparingly I think. It's very hard to get eyes in focus with about 5mm to play with (but looks amazing when you actually nail it, ears and nose blurred, eyes tack-sharp).
I'd like the 70-200 f/2.8 L as the final part of the kit It gets rave reviews and completes the camera bag. Pricey though.
Below, an artsy image of my new "Breeze Block" (seriously.. it's a heavy combo..reassuringly so)