Well 90mm is a little long for portraits on a crop unless you really plan shoots (i.e. travel to fields etc.) but then again other macro lenses don't really have the DOF needed for portrait lenses, particularly not on crop sensors.
I'd say it's a good buy. It'll be sharper than the portrait lenses at f/2.8 and you have the flexibility of being able to do macro as well. You won't have the extra bokeh that you would from having an f/1.8 or larger (smaller number) aperture, but on the D3100 you don't really have choice; the f/1.8 Nikkor 85mm won't autofocus on your body, and the cheapest 85 that will is the Sigma f/1.4, which costs over £600.
If you find it too long, consider the 50mm f/1.8/1.4 (must be AF-S, or Sigma) to round it out and you'll have a very capable portraiture set