People make a big deal that it's only f/4, but I have absolutely no issues using my 24-105 in low light. It's sharp enough wide open, and the high ISO capability of the 6D helps me out as well. Plus for static objects, the IS works very nicely.
The f/4 aperture doesn't bug me for low light, it bugs me because it's not ideal for isolation in most situations. When you're used to shooting with primes that all shoot at f/2.8 or faster, f/4 seems extremely slow. That's the main reason my L usually stays at home, otherwise it's definitely a great lens, most people use it more than any other, I just get bored with it.
As for the 85/1.8; I just bought one, so far it seems pretty nice. The build is excellent, so is the AF. I had the Bower (Samyang) 85mm first and had to return it due to a sticky aperture and bought the EF because of the sale on Amazon, I'll see how it fares against the Samyang after I give it a real shoot this weekend.

