This is where it can get very subjective.
It's much more 'creamier' and 'buttery'. You've got to be a proper bokeh nut to appreciate it. I never saw the fascination until I used it side by side with the canon. Then my eyes were truly opened!
I've sold the siggy on our site to raise funds for my wedding. However, I remember doing a comparison with the canon. Let me see if I still have the comparison pics and I will put it up on a flickr account I created a few weeks ago.
Busy with wedding prep at the mo as I've only got 2.5 weeks left before the big day, so if I forget, don't hesitate to pm me.
In the meantime:
shots from real users
http://www.fredmiranda.com/forum/topic/661966/0
http://www.fredmiranda.com/forum/topic/716880
ok, i found some sigma pics on my pc. The majority of them have my nephew and niece, and I don't want to upload those. Here is a pic from the sigma:
http://www.flickr.com/photos/45859795@N05/4293877242/sizes/m/
These wooden shelfs 2.6metres long. On the far right are heavily chequered curtains. When I took the same shot with the Canon at the same settings, the bokeh was fuzzy and cluttered. The siggy 'smears' it all over the place and I personally find the siggy's bokeh way more attractive.
I never understood bokeh until I got the siggy. I got it confused with shallow apertures. But I now understand that the 2 are completely different.
Ok for completeness sake. Here's the one with my nephew and niece blanked out.
http://www.flickr.com/photos/45859795@N05/4293910404/
If you look past the 2 kids and near the top middle and top right, you will see 2 metallic objects. On the far right is a BIG metallic pot (used for cooking). It has a rough outer surface and many 'ridges'.
when taken with the canons, this roughness is more pronounced and the various tones are shown. The sigma however, really diffuses and smears the colours and light in circles.
Like I said, it's hard (for me) to explain. But those that have used it extensively (like those in the links above) get it. Don't forgrt the better build quality (way better balance on a bigger body), sharpness and almost no vignetting.
There's a reason why sigma choose to sell it at £50-£70 more than the Canon f/1.4. They'd be really silly to sell an inferior lens for more money.
I am a massive believer in taking advice from those that have used lenses rather than those just expressing other people's opinions/or what they have heard. A lot of forums spread issues about focusing and this or that and other problems when in reality the numbers of real-life issues are low. Hope that makes sense.
I forgot to add: my siggy was not as good as the canons when using the outer AF points on my 5D. However, this was not a problem for me as I mostly used centre af point.