I had a pretty old one (before the EX or DG designations came into force if I remember rightly) on my 5DII, it was a bargain and needed a bit of tweaking from Sigma, but when I got it back I was pretty happy.
It wasn't the sharpest lens wide open, but it was very usable. Stopped down I was very happy. For me the biggest annoyance was the 82mm filters, so I moved to a Tamron 28-75 so I could use my existing ones. It certainly wasn't as good as a Canon or Nikon 24-70, but for a quarter of the price it was never going to be!
In response to your last question, it's a huge 'NO'. Sigma lenses are certainly not low end products! Some are, and some aren't, much like the bigger names. The lower-end lenses like the 24-70s and the 17-50s are usually not quite as good as the bigger names, but are much cheaper. Some are better than the Canon/Nikon equivalents (the Sigma 50mm f/1.4 comes to mind straight away). Some sigma lenses don't have equivalents from the big names either (like the awesome 120-300 f/2.8).
Of course they've made some absolute shockers over the years too though, and I don't think their quality control is as good as Canon or Nikon, but it's nowhere near as bad as the internet would have you believe
Chris
If you need further advice that Sigma aren't just low end, have a look at one of
these