My first Sigma lens was the 70-300 APO DG, which was great for the money, no problems at all. Then came the 10-20. I must have been very lucky because I've had this lens for 3 years and it's been brilliant since day 1.
Then I bought the 120-300 2.8 to replace the 70-300 and I must say that I am a big Sigma fan now. Have bought the 1.4x and 2x TCs, and I'm happy with both. I even use sigma flashguns, having bought the Sigma EF500 DG Super a couple of years ago.
Have used the 30mm 1.4 and was amazed by the IQ, built quality and results, have tried the 50-150 2.8 in store, and the photos where great on my pc and I was quite happy with the quality by "birdie's" sigma 70mm macro when I tested it last time we met.
All in all, I have owned or tried 9 different pieces of equipment with 100% satisfaction. And tbh, for the price difference in some lenses between sigma and canon, I am willing to take the risk of having to send it back and wait for a couple of weeks. But I always buy in-store, tell them to hold it for me, go home, check the photos on my pc and then go back to store and buy.