I had previously had the iPhone 3G so knew pretty much what I was getting when I decided to upgrade to the iPhone 4. The styling of the 4 is much more contemporary, slick straight and flat lines as opposed to the curving edges of the 3. New features include a flash on the camera, front and rear camera, video recording, facetime to name but a few. I have had a few problems however.
The right hand part of my screen is non responsive to touch, and therefore I have to keep turning my phone to use the text option. There are certain apps etc where the screen cannot be turned and so therefore I am unable to use these (adding new contact etc, I cannot type in certain letters).
Despite this, I will still happily choose and iPhone when the time to upgrade comes round again. I feel they look and work better than any other smart phone out there, hence why most (i.e. the samsung something or other) are trying to look just like the iPhone and work the same way! The screen is by far clearer than that of a blackbury and there arent those pesky little buttons to contend with.
iPhone all the way!!