Hector Javkin , Dec 15, 2008; 09:26 a.m.
It would help to know what kind of photography you do, whether you have older Nikons, why you bought the D90, and whether you consider a large flash inconvenient. (For example, if you decided to get the D90 rather than the D300 because the D90 is smaller, you might hate the SB-900, which is really big.)
You should look at http://www.bythom.com/index.htm . He reviews the SB-900 somewhere on his site, and he knows Nikon equipment very well.