I agree with others that while this is a 'minor' update, it's still very impressive.
The 3GS is my first iPhone. Quite frankly, while the previous models were cool, I never bought one because to me, they hadn't yet achieved a sufficient level of polish that makes for a great user experience. The storage capacities were too small, the camera was marginal, there were oversights in the OS, and it wasn't the snappiest of machines. The app store was still in it's infancy, or didn't yet exist at all.
But now? My oh my. The 3GS combined with OS 3.0 makes for a great phone. The speed with which I can open and switch between apps is very impressive - I'm never waiting. The camera is finally workable - I'm actually interested in taking pictures and recording video with it. There's 32 GB so I can achieve a decent balance of apps, music, and video. The app store is unparalelled. On which other device can I run useful or unique apps like Brushes, Ocarina, RJDJ, Newsstand, Shazam, and Photogene? It's got everything! Finally, I never imagined I would grow to love the body design so much. It's comfortable to hold and easy to use. The size and weight feels very natural.
I imagine the 3GS as the first 'complete' iPhone. Previous models always seemed more like work-in-progress devices. I honestly can't find any complaints to make. I can only imagine how much better this device will get as the years go on.