Just as good as any other camera...
With the D90 you still have Auto mode and a bunch of defined presets (night, portrait, sports, etc). You can venture out as far as you'd like and still fall back on any of those modes if you get stuck.
I started with a D40, (still love that camera) but feel that the D90, although more bells and whistles, would be just as easy to learn on with the added benefit of growth potential.