I have not received an iPhone 4, but I was able to download iOS 4 on June 21st. It took 20 min to download, install and sync. Others reported different download and install times. It depended on how the download was started and the amount of applications that need to sync.

It took a while to get use to using, but iOS 4 is done very well. Below is a hands on video that can explain the features much better than I can. My favorite parts to the new OS include the following:

• Digital zoom with the Camera, being able to zoom in was certainly missing.  Now can we get some better focus.
• Separating my apps into folders and reducing my pages to 2 is great.  It will take a bit of getting use to now that they are in folders.
• The new iPod controls with the swipe is great as you can lock the screen position.  Being able to keep the iPod in one position while I drive makes me a better driver.

I used my 3GS with iOS 4 today at work, and despite my best efforts to conserve energy, I was down to 68% by the end of my 8 to 5 shift. My use consisted of checking RSS, Twitter, Facebook, Mail, and iPod. The iPod ran most of the day, but I made a few phone calls. The battery life, or lack there of, is not a deal breaker for the new OS.  Being asked every time I first use an app if I want to allow location based service is wearing on me.   Of course many of my apps are not fully ready for multitasking. I can’t wait for the Slacker App to have multitasking.

For a free upgrade I am very happy with the new OS, but battery life may force my hand to buy a Mophie Juice Pack. In the end depending on your iPhone or iPod Touch or how you use your device your milage may vary.  Enjoy!