At the recent Focus of Imaging I upgraded my Nikon D80 to a D90. I chose the FoI as deals were available and I purchased D90 with Sigma 18-55 f2.8-4 AF OS lens. I sold lens on eBay immediately and offset some of the cost.
The first thing I noticed was how good images were at higher ISO settings, even up to 1600 ASA. Other things I noticed quickly were
Faster continuous shutter operation.