I've had two Sigma disasters:
30mm f1.4 DC. Fine on my 7D with +18 MFA. Not usable for stills on my Rebel. Lovely images for video, it really hurt to send it back.
17-35 f2.8-4 EX DG. Just all round grimness. Ghosting, focus shift. Returned and got a 17-40 f4L instead.
Had some joyous Sigmas too though:
My 18-50 f2.8 DC is a great alrounder on my APS-C bodies. Delighted with it. I bought it used and it seems it had been dropped before, but a rebuild gave me total confidence in it.
My 70mm Macro, on the rare occassions I use it, never fails to delight.
I liked my old Bigma 50-500, just need faster lenses for video and wouldn't carry both this and my 70-200, so made a call.
I loved both my 10-20 and 12-24 when I had them. The 10-20 was a bit of a pain because square filters reflected what was behind the camera into the shot (no hood with square filters) and the 12-24 was rear filter only but no issues with focus, build, reliability, all EX, and well put together.
When I used MF gear I had an old 28mm f1.8 in MD fit, which was a treasure to use, especially with cross-processed agfa 100.
They make a lot more lenses than most companies, and the designs are largely adapted per mount than unique to a system, but they push the boundaries.
I hope the new ethos and the ability to firmware update etc makes them more consistent. They are imaginative and plug holes in other folks ranges. Can vouch for those who've had problems, can vouch for those who have had great experience.
In contrast, every Canon item I've had has been excellent (apart from my 7D's UDMA issue, 7D's grip issue, XL1 back focus issue, 18x dead sensors in MV600 camcorder... etc etc etc)