Lens Rentals published their data for defects and repairs across the kit they own. Some interesting nuggets in here. Sigma’s 120-300 f2/8 stands out as a serious lemon with 3 times more failures than the next worst offender. This would certainly make me think twice about this lens but in general Canon and Nikon also feature heavily in the top 15 so I don’t think it means avoid Sigma in general.
Check out the methodology and the list at: http://www.lensrentals.com/blog/2012/09/lensrentals-repair-data-january-july-2012