To those hoping that these superteles will be half the price of Canon, I doubt that: cheaper, undoubtedly, but my guess would be about 75% of the price of the Canon equivalent. Sigma currently has a 500mm f/4.5 which B&H sells for $5,000, slightly below half the price of the faster, lighter, optically superior, IS-equipped 500mm f/4 IS II. You can be sure that the new Sigma 500mm f/4 OS will not be $5,000 - more like $7,500 would be my guess.
So, to the guy about to pull the trigger on a Canon 400mm f/5.6, asking if the new Sigma 400mm f/2.8 is likely to be in his price range, well if your price range doesn't stretch to about $8,000 then probably not.
Regarding weather-sealing, if ever there were products for which Sigma should commit to proper weather-sealing, these new superteles are the ones. Honestly, I'd be amazed if they didn't get that part right.