Looks nice, but it only complements OSM if the pictures and db are released to all under an open licence. I can't find one on the Mapillary site (though I may have missed it).
Only if you have an iPhone.
I'm able to view the pictures just fine using a desktop web browser.
I guess you mean the pictures are taken by iPhones.
Yes. The point of being like OpenStreetMap instead of Google Maps is the way the maps are not read-only, but that anyone can contribute. And for Mapillary, this is not true unless you have an iPhone.