Also, although I haven't done any real radio navigation yet I have read a couple of books on the subject and it seems to be a fairly good representation of what I expect to find in the future when I eventually give it a go.
One of the things that I found FSX very good for was anything that had to do with IFR flight. Flying on instruments, but also all sorts of radio nav.
The "slew" mode (where you can just position the aircraft wherever you want in the sky, independent of pesky things like aerodynamics and gravity) is very, very useful for understanding what the different needles (ADF, VOR, ILS) do in certain situations.
I also used the autopilot a lot when doing IFR flights so that I could concentrate on the radio navigation part. Doing NDB holds with crosswinds for instance, with the AP in HDG mode.
Oh, and you can setup and fly an ILS approach, first on autopilot with nil wind, then manually with nil wind, then with a howling crosswind. And break out of the clouds - coincidentally... - exactly at minima. You can't choose those conditions in real life.
But as far as pure flying skills is concerned, no. It's not just a matter of the "seat of the pants" feeling, or the view to the sides that's missing, but also the flight model of FSX is not very good at the edges of the envelope. Particularly the stall/spin behaviour is nowhere near realistic.