I wouldn't be surprised if this new Sigma will surpass the Canon L glass in terms of performance. Both their new 35mm and 24-105mm have done just that. I think if this 50mm can get within 80% - 90% of the performance of the Zeiss 55mm Otus.. That would be quite a feat.
Forget about the price of the Otus for a second.. How many reviewers have claimed the Zeiss 55mm Otus as being the best DSLR lens E-V-E-R?
Quite a number of them. I'm thinking the Sigma 50mm will come in around $750 considering the 35mm version comes in around $900. And if you think you're going to get a Zeiss 55mm Otus copy for $750. It's not going to happen.
Would the Sigma be worthwhile if it has 80 - 90% of the performance of the Zeiss Otus at their price level? Sure! A 2014 Mustang/Corvette are hot cars and totally great. But they are not Ferraris. Whether you like Ferraris or not. There is a reason why you pay a premium.
And that's a great comparison. The lenses will be for two different markets. A pro photographer making $100k+ plus a year shouldn't sweat about a $4k, near-perfect lens. Especially if it's used often for the next 5 years or more. The pro should make an easy $30k a year from using that lens.
I love Sigma for coming out with amazing glass and going for the best. And all for a very reasonable price.