After finally giving up with the Canon 50mm f/1.8 and its notoriously unreliable focusing we decided to upgrade to a newer fast 50mm, this time to a lens with notoriously unreliable focusing. Yeah.
It's not as mad as it sounds. The Sigma 50mm f/1.4 is a very good lens, with solid build quality, fast aperture, creamy bokeh, very low distortion and reasonably fast and quiet focusing. As usual it comes standard with lens hood and sturdy lens case (take note Canon!). The centre-pinch lens cap is easier to take off but I find it quite fiddly putting it back on again. This may be due to the filters that tend to live on the front element.
We agonised over this lens for a long time because despite all the good points and relatively low cost compared to similar quality lenses this lens has had a reputation for unreliable autofocusing. This isn't just being front- or back-focusing, which could be compensated for with MA, but more random front and back focusing which can't be compensated for.
This is something that is quite common with 3rd party lenses. As they haven't licensed the Canon autofocus routines there is always a concern that they're not as accurate or reliable as Canon lenses. Things tend to be fine when manual focusing or using Live View but that's not always appropriate.
Ultimately we took a gamble and got one. It's a slightly newer model (with smooth plastic barrel rather than Sigma's distinctive matte coating) and the focusing seems to be ok. Being a lens with a very wide aperture and consequently shallow depth of field, it does require some care in use to keep things in focus. I still have slight doubts about the lens but also doubts about my ability to use it properly and my expectations wide open at f/1.4 are probably unreasonable. When it gets the focus spot on the results are excellent and I don't feel I need to treat the lens with kid gloves, like the similar Canon offerings.
Sigma do offer much longer warranty periods than Canon and seem to have good customer service, so if you do get a bad one it should be easy to get it fixed. Sigma offer some very good lenses in their range, it's a shame that it's such a gamble buying one due to what seems to be poor quality control.