Not all change may be an improvement.
Apple's new iPhone 4 seems better in many ways than the old 3rd generation iPhones. Lots of neat features. But, as reported in so many places, it has not been without its problems.
My own issue with OS4 is that it has made my older 3G iPhone incredibly slow. I'm guessing that the firmware is simply too complex for the older hardware to handle smoothly. "Apps" simply stop working or have incredibly slow response. I'm not alone.
I have not "upgraded" my wife's iPhone to OS4 and will probably not do so based on the experience with my iPhone.
More here.