I've spent the month getting cloud assets into Hammer and it all works great. You can drag models from asset party right into the map. You can even open the browser and drag straight from the website into the map!
The assets that the map uses are saved with the map file, which means that when you save the map and return to it in 6 months, opening the map in Hammer will automatically grab all that stuff so it'll continue to work.
We're going to be looking at getting other asset types in over the next few months. Things like materials, prefabs and sounds are what we have in mind.