I struggle to see the benefit of this over many other options for landscape. While it's nice to pick out features with a longer lens it's an odds focal length for that and at f/1.8 it's overkill aperture wise as well. It's too long for astro and night photography where a fast aperture would be useful.
Portraits though, I guess it would be great, which is where I assume it'll be marketted, or as a complimentary lens to another Sigma DC lens to cover all ranges.
EDIT: Actually I change my mind a bit here. my 17-55 is oftentimes too short for travel shots and as you mentioned in an earlier post the combo between the 18-35 and 50-100 could be great for travel.