On the weekend I rented, among other things, the well regarded Canon 85mm prime. This was one heavy lens, and while capable of fantastic background blur, I didn't really think it through. I was planning on shooting against a white background (not too hard to blur that!) with some high powered strobes. When I got out the f/1.2 lens I couldn't get the lights low enough, or far enough away to use the wide open aperture. At f/8 this is like every other 85mm L lens that I own.
I did manage to capture a few available light shots near windows, etc. and they turned out great, but I highly recommend renting this for available light shooting, not studio!