As far as I know, I'm the only one that really makes any use of it though I would really like to see that change. Besides OutpostHD, I'm using it with several internal projects and a map editor I recently released: http://lairworks.com/projects/landlord-tile-map-editor
Note that that map editor hasn't yet been updated to NAS2D 1.3.1 and is missing several key features supported by a forked, private version of it being used for a different project.
There are three things I'm particularly happy with:
1) I fixed some long standing bugs that I've been scratching my head over for years.
2) I rolled TinyXML directly into the NAS2D interface and gave it a _very_ much needed interface overhaul (holy crap that library is a lot larger than expected and supports far too much to really be called 'tiny' so I pulled out the unnecessary stuff and will eventually comb over the remaining code and port it over to C++11 standards)
3) I cleaned up a lot of the public interface for NAS2D and removed as much of the implementation details as I could. There are no longer external library interfaces exposed in the public headers, the namespace has been cleaned up and it just all around feels a lot better than before.