It depends on what you want to shoot. In general, 85mm is a portrait lense for closeup to quarter body. If you want to shoot this kind of portraits, I don't see why it does not make sense to get it.
And I presume the Sigma 105 f/2.8 is a macro lense?
Thank you for your replies. I shoot mostly with a 28-105, 28-80. Basically anything. Haven't really 'specialised' yet. Heard n read many good things abt 85/1.8. Does the 105/2.8 macro fill in the functions (namely, portraiture) of the 85/1.8? I'm thinking of using it with the D100 to take pix of my daughter.