My Sigma lens experience is as follows:
1) Bought the highly thought of 18-35 F1.8 crop lens (did this for my C100 with Dual Pixel Autofocus for low light situations. ALso had a 7D2 at the time so I fancied it as a fast indoor lens. The lens was fantastic on the C100 for video (but with no IS so it couldn't be handheld - I so wish they would bring out an IS version - so I ended up using it only very sporadically). For pictures, it just was so unpredictable - missed 40% of the time. I sold it when I unloaded my 7D2 and got only 50% of what I paid for it - so I do not agree that Sigma lenses hold their value better than Tamron lenses - and my local camera store in Toronto, where I am a very good customer, quoted me what they buyback Sigma lenses for and it isn't good.
2) When I purchased two 5DSRs, I did a major, major, major lens overhaul of all my lenses and ended up buying the Sigma 24-105 F4 OS and trading in 1/2 my other lenses (all my Canon Primes). I can confidently say, as I have tried basically every EF mount lens, there is no better lens, consistently better, on a 5DSR, than the Sigma 24-105 F4 OS - it has never, in 1000s of shots, missed a single shot on the 5DSR - even with outrageously low shutter speeds - just to see if I could do it sometimes. I understand this is an F4 lens, and thus can't be widely used by event Pros, but I am praying, seriously praying, that the upcoming Sigma 24-70 F2.8 OS (and I think the OS is the key here), can deliver the way the 24-105 does.
I have never heard of anything but problems with consistent focus with Sigma Primes - and this is from photogs that I know personally who shoot weddings - and for the most part, all eventually ditched them because the consistency ended up driving them crazy; and the local buyback prices, at least here in Toronto, would confirm that. Yes Sigma makes sharp lenses, and I think for enthusiasts they are a great choice, especially for cameras like the Sony on sensor phase detect A7Rii where you never miss focus (unless the object is moving quickly towards you), but outside of the Sigma zoom 24-105 F4 OS, I wouldn't buy any of them for high volume paid work.
Hockey and wedding photographer. Favourite camera / lens combos: a 1DX II with a Tamron 45 1.8 VC, an A7Rii with a Canon 24-70F2.8L II, and a 5DSR with a Tamron 85 1.8 VC. Every lens I own I strongly recommend [Canon (35Lii, 100L Macro, 24-70F2.8ii, 70-200F2.8ii, 100-400Lii), Tamron (45 1.8, 85 1.8), Sigma 24-105]. If there are better lenses out there let me know because I haven't found them.