I use it with TestDriven.Net (which obviously can also use nunit) and like it a lot. There are the reports that it generates, which ultimately are not necessary, but make it "nice" to find your failed tests.
It has a whole set of database testing helpers (like Rollback) which I haven't used yet, but I don't think nunit has those.
At the core, there really isn't much difference.
The one bad part is the documentation for it is in a wiki, which I find very hard to navigate.