The scroll control is limited to small, medium or large increments, which in the TextWrangler text editor amounted to one, three or six lines of text, respectively. In Photoshop, with a document displayed at full size, the increment was 16 pixels no matter what. I'd have preferred a greater degree of control - and a wheel that was not only as quiet when pushed forward as when rolled back, but also one that felt smoother.
In use, it felt good, whether in Photoshop, Finder, text applications or games. Inevitably, you'll want to tweak the rodent's sensitivity to meet your personal taste, but Mac OS X's own System Preferences pane provides sufficient scope for that.