This is certianly an interesting addition to the range of lenses that will be available to us.
Personally I do not consider it to be an ideal portrait lens as the working distance will be greater than available in the spaces I now work in. Also the perspective may be flatter than we expect to see in portraits.
The low light capability will make it an ideal lens for the stage and indoor sports as has been stated.
More than anything, the Image Quality will always be the key factor in determining the use of this lens, and if the 12mm and 45mm are anything to go by, this will be true asset.
I look forward to trying this one out, but as has been the case in the past, having tried one, I end up getting one...