Thanks for the info everyone, the "Russian roulette" analogy convinced me I will wrestle my buddy to the ground and drag him away from the Sigma cabinet if I have to.
You'll be doing your friend a disservice.
The Sigma is much sharper then the Canon 50 f/1.4 at wide apertures, has better bokeh, and has much better build quality. The bokeh is amazing and the lens is actually usable wide open. I personally think it's the f/1.2L's equal. Some have AF issues, but if your friend is shopping in person then he can cherry pick the lens he wants on the spot. FYI, the AF motor in the Canon version is famous for having accuracy problems or just breaking over time.
I'm quite frankly surprised at the number of complaints here. Usually when the Sigma 50 f/1.4 comes up in forums the majority of reports are good.
As to AF issues...no doubt some units ship out of alignment. But I can't help but think that many complaints trace back to people struggling to use fast primes, especially since I've heard the same types of complaints from people who went with the Canon f/1.8 or f/1.4, and even the f/1.2L.
* f/1.4 is difficult for the AF system. This is true of any lens. DoF is stupid shallow at f/1.4 even on crop. On FF? Try one eyelash in focus.
*
Any forward/backward movement on your part at wide apertures will move the focus away from where you want it. By the same token you cannot focus/recompose at wide apertures.
* Focus point indicators in the VF are not perfect. Usually the AF sensor points are larger than indicated. If the point is over a target which is not flat, the AF system may focus correctly but still focus somewhere you don't want. (DoF is that shallow.) 3D target areas that work just fine at f/4 or even f/2.8 may not yield the desired result at f/1.4 or f/2.
On a tripod pointed at a flat target the Sigma is about as consistent and accurate as the Canon 85 f/1.8. Perfection cannot be expected with super fast primes however. If you defocus, AF, and shoot 10 frames in a row, at 100% it will be clear that some shots are better than others even in this simple test. (I didn't have access to a Canon 50 f/1.4 when I did this. The f/1.8 is much worse than either the Sigma or Canon 85.) This is with a 7D which has an even better AF system.
I love fast primes, but all of them require some care in AF. There's something to be said for split screen manual focusing when using fast primes, and I've considered a KatzEye for this reason.
With that out of the way...I've shot kids (always in motion) with the Sigma f/1.4 with good success rates. And when you nail it...well...30" portrait prints at f/2 and f/2.8 are no problem at all, tack sharp and gorgeous.
