Sigma is taking a new approach to marketing and they say, in their quality control. This new model 120-300mm is weathersealed and may be a half priced alternative to the Canon 300/2.8, for those in the market for that sort of lens.
The Siggy 85/1.4 is a very good lens and I would expect the new Sigma35/1.4 to give the current Canon 35/1.4 a very good run for its money as well. They will always beat them on price.