Short answer: yes.
Longer answer: Absolutely!
Hillbilly answer: You betcha!
Beancounter answer: I purchased my first iPhone 4GB (2G) in Oct 2007 for $300, direct from Apple. Today, I sold it on Craigslist for $225. I purchased my 16GB iPhone 3G for $300 in July 2008. That phone is about to get sold on Craigslist as well, for about $375. My cost to own for both iPhones is $0. I expect to sell my 32GB iPhone 3GS next year, for more than I paid. It’s an unbeatable deal.
Geek answer: The combination of a faster processor and more RAM makes a huge difference. I would bet the RAM is contributing more than the faster CPU. A good analogy would be using OS X with 1GB of RAM and then upgrading to 2GB (just enough). With the memory pressure relieved, nearly everything is more responsive.
The previous iPhones lagged when switching apps, [re]loading web pages, and especially when taking and saving photos. All those little pauses are gone. Switching back and forth between apps is nearly instantaneous. That alone is worth upgrading for. Seriously.
But I upgraded for the better camera. The previous iPhone camera was quite poor. The 3GS camera is not yet good, but certainly better.