I find myself lusting after this lens to use it on my 5D2, what with it being the Rolls Royce of portrait lenses. At the moment, I 'only' have my 70-200 f/4 IS to cover that focal range.
I used to be addicted to wide apertures, but am slowly recovering - as illustrated by my recent purchase of the f/4 zoom above. Yes, I could have afforded the 2.8 IS, but no, I just didn't want to carry that monster of a lens around. I accept (with a tinge of regret that I couldn't fully smother) that this restricts my shooting options somewhat.
Anyway, like I said I'm looking at the 85 f/1.2 thinking I won't ever need another lens after that, because I'm prone to lying to myself. However, I just realized that while it's compact and looks funny, it's also a hefty 2.25 lbs - more than my 24-70 which I already find to be well in the 'heavy enough' category.
Now, slow AF I can live with. High price tag as well (somewhat blunted by the fact I expect to get it while traveling in Hong Kong). But I know that if the lens weighs half a ton, I just won't want to take it with me.
I know you're supposed to stop it down for optimal use, but that's not the primary reason I want it. I wanted to use it like I use my 50 f/1.8: lightweight lens I tack on when I go to a bar or a club or some event in dimly-lit conditions, and where I won't want to stick a Speedlite on top of an already conspicuous camera. So, in the crowd, low-light, no flash. Hence my salivating over the f/1.2 here. But lightweight, it seems, isn't going to happen.
Do I wish I could get more light in my 50 f/1.8? Yes. But in general, it's good enough - just barely, but good enough. So I've started looking at the simpler, lighter, cheaper 85 f/1.8. It may be the better choice for me, as I find that my 50 is a little short, and more importantly that the AF on that lens is just terrible.
All of which brings me to my question to you: how does the 85 f/1.8 perform in low-light conditions, such as f/1.8, 1/100th of a second, ISO 6400? Is the IQ terrible in those conditions? Will the AF (remember, 5D2 here) be as slow and unreliable as with the 50, or can I expect better - if not perfect - performance on that side?
Sorry for the long-winded post, and thanks for the help! 
edit: almost forgot: would the Sigma 1.4 solve all my problems? It's also a bit heavy, but still more manageable than the 1.2...




