I take a neutral view and kinda expect to see regular problems reported with both Canon and Sigma. You see fewer from Tamron and hardly any from Tokina. I do wonder how amny are user error and how many real problems. Who knows?
Nitsch is spot on and we'll no doubt see some reports of problems in this thread, many of which are churned out a semi-constant basis of examples of ... whatever.
Last week, after a period of bashing a specific Sigma model I posted a poll for owners or past owners. To date 31 owners are delighted and the only 3 anti's had never even owned the lens!! This is what happens. People are biased. Some feel they have to save buyers from the 3rd party devil.
I've owned more Sigma's than Canon's, but for no reason other than the ones I've bought have offered better or similar performance and higher value and equal resale value. At the longer zoom range I have two Canon L's because they offer better performance and value ... and because they have IS. Sigma has one OS lens. If their 100-300 f4 had OS I would possibly buy that instead.
Do the research, ask for owners views, be clear about your budget and buy what you feel is right.
One thing to bear in mind. I have been very impressed with Sigma customer service here in the UK. When I've emailed about some old lenses and compatibility with newer DSLR's they've be responsive and honest. I sent my Sigma in to be checked as I felt it could do better wide open at 24mm and they said up to 21 days. I received it back max. 10 days later and it seems fine now. No charge of course.
Both the Sigma and Tamron are fine lenses. If you want a Canon then spend 3 times the price. It's a fine lens too and as you'll find from posts over the last few months you're not guarenteed anything going that route either.
Good luck ...