We stayed there earlier this year. I really liked the area and it wasn't hard to get into DC at all. We didn't eat at the hotel, but found other restaurants nearby that were good. We walked around the harbor one night after dinner and it was wonderful. We did not have a car and relied on public transportation to get to/from DC. Pretty good experience in that regard.
Having said that, we were there on graduation weekend/air show, and scheduled maintenance of the metro. On one day (graduation), hardly any of the trains were running system wide and it was quite the nightmare for everyone. That's more of a DC Metro problem though, not National Harbor.