I've had every model iPhone since the first day they came out. Even though the 3GS was a significant improvement over the previous models, the battery life sucked. I wouldn't consider myself a "heavy" user, if anything moderate Internet use. But I'd only get one, maybe two days of battery life out of my 3GS. But with the new iPhone 4, I'm getting 3-4 days of use before a full charge is needed. Same usage patterns and settings, but a significant improvement in battery life.