I've had my iPhone 3GS since they first came out in 2009.
When iOS 4.0 came out, I hated it. Slow, drained battery, and tons of bugs with Safari, Messages, etc.
I'd like to have a comparison of 4.1, 4.2.1, 4.3.3, and 5.0 on here soon.
For iOS 4.x and 5.x, I recommend installing KillBackground from Cydia (free). It gives your device a simple, 1-button press to close all background programs, quickly freeing up memory and allowing quicker application launching and smoother performance.