I upgraded 2 products to the new OS, my 2G iPod Touch 32G and 3G iPhone 8G.
First was my iPod Touch. I synced it with iTunes 9.2 and initially iTunes stated that the OS was up-to-date. After pressing the check for update button I was then asked if I wanted to upgrade to iOS4. I selected yes. First iTunes downloaded the update then asked me if I wanted to install, yes of course. iTunes backed-up my Ipod Touch then installed the new OS. Then restored my settings from the back-up. This process took 90 minutes. After it was done my iPod Touch was not longer connected to iTunes and iTunes was frozen. I had to force quit iTunes in order to get things back up again. Before re-syncing with iTunes I checked out the operation of the iPod Touch. Everything appears to be working as before. The new Mail app is much better with the choice of handling multiple email accounts.
Next I upgraded my iPhone 3G (no S). Once connected iTunes 9.2 immediately told me that a new OS was available. I selected yes to all appropriate menus. First iTunes downloaded iOS4, next backed-up my iPhone 3G, installed new OS, then finally restored my settings from the back-up. Same process as my 2G iPod Touch. Except for the time. The iPhone upgrade took over 3 hours. I had the same frozen iTunes after all was done making me force quit iTunes again. My Phone 3G appears to be working as expected. No problems. I really do like the new Mail app. I have not tried using folders yet.
As for battery life , maybe a little more. The current battery life before re-charge seems to be very low to me. Starting with a full charge in the morning, I check email and read news on a break at work thats about 15 minutes. Then on lunch I check email again and read more news and sometimes play a game, that lasts about 30-45 minutes. When I get home and check battery life I am down to less than a quarter of life left. Basically I get about 2 hours top between battery re-charge cycles. That seems to me to be way to low. SInce I carry multiple battery rechargers for back-up when I am way from home I am not concern yet. Of course I will be getting a new iPhone 4 next week. We will see how that goes.