This is a screen from my iPhone running IOS 4 (taken by pressing home and power on/off). I have a 3GS which supports firmware version 4, as does the new iPhone 4.
First I had to update to Itunes 9.2 and restart my PC. Then downloaded and installed the new firmware. Then I started playing.
You may have noticed the background on the home-screen is different, this is the new default wallpaper which can be changed by simply going into 'settings', then 'wallpaper'.
I went into email to have a look and the iPhone now has threaded email just like Gmail. I also had a look at the spell-checker which is a nice new addition as it has recommended spellings when you highlight a word that is red.
I then decided to try to create folders. I held down the Youtube icon on the home screen until it 'jiggled' then dragged it on top of Google Maps to create a Google folder.
I was then going to try out the new iPod controls but when I went into iPod it had deleted all my podcasts (94 of them) which is a bit odd because it kept all my applications I had downloaded.
There are a few reports of applications that have problems with crashes and the makers will have to update these but so far the ones I have tried have all worked. Another problem though is applications running in the background. When you open an application it continues to run after you have exited and you need to know how to close these so they do not take up all the memory or cause crashes, freezes or excessive data usage. Double-tap the home button and a menu of things running should open up (see screenshot below)
Then hold the application you want to close until it 'jiggles' and a red '-' sign appears. Press it to close it. After a few minutes use I had 8 programs running in the background. Why you have to close applications this way is a bit of a mystery and seems a bit of a hassle. Maybe an app that does it for you will be on the way.
First impressions are quite good. Apple have added a lot of features I had on my android (multitasking, threaded email, wallpapers, folders) and it is still looking smooth. No sign of slowing down or crashes. I also tried the zoom on the camera. It is a nice touch but display goes blurry and I will probably edit the pictures I take in software.
There is a list of applications updated for IOs 4 here.