I've been planning to buy a walk around lens for my 7D for a while. I've had my 24-105L for about 5 years and although its a fine lens, its not wide enough on a crop. Besides, I've also got an EOS 3 35mm camera and the 24-105L was made for that and lives there most of the time.
I really needed one of the 17-5x lenses for my 7D. I felt that the EFS 17-55 was much too expensive and poor value for money. I mean, 760 UK pounds and you don't even get a lens hood
Then there is the dust and possible IS failures. Only one year warranty too 
The Sigma 17-50 is much better value for money at 480 UK Pounds IMHO, particularly with its 3 year warranty. But there's the well documented Sigma QC issues. The problems may be exagerated but undoutedly Sigma owners are more likely to have to return their lens than Canon owners.
Well I decided that the price difference was worth the risk and I bought the Sigma 17-50 f/2.8. It arrived on Friday and after testing it over the weekend, I'm pleased to say that it seems fine and I've got a good copy. What a relief 

