I installed iOS 5 on my iPhone 3GS. It was a rough start, but I am happy with the end result.
For the last several iterations of the iOS on my 3GS has really slowed it down. The latest 4.3.5, slowed down my phone so much that I was thinking Apple was purposely slowing down the 3G series phones to force them to upgrade to the 4S. Unlike Microsoft that benifits from speeding up old hardware, Apple requires new products to be moving all of the time to keep their stock price up, and their company solvent, as no one can install their OS or use their software on anything other than Apple provided hardware... and Jobs said that Bill Gates was too uptight.. So slowing down older hardware would be right in their best interests, and therefore not a large logical leap to assume it was happening.
With iOS5, I found that my phone not only performs much faster, the battery life and the signal strength seem to have improved as well. I can't yet tell about calls dropped, as I have not had the time to really test that particular annoyance of the iPhone out completely. Weather this is simply a new algorithm used to compute the signal and battery strength I have no idea. I do remember seeing my signal strength greatly improve when I installed iOS4, only to be disappointed later when I found out that Apple was displaying strength greater than what the strength really was. This could be more of the same, but I want to believe that Apple has improved their software and thus found ways to improve their power usage, and to detect signal.
The new features are very welcome, especially the new camera features. The camera can now be accessed from the locked screen and you can take a photo by pressing the up button on the volume hard button. Infinitely easier to take a photo of yourself using this method than trying to find the soft button. I would always miss and end up in the photo album...
The new notification center is also very cool. You get a plethora of app data in there accessed by simply pulling it down from the top where the clock usually is. I know this is a feature they stole from Android, but it is a welcome addition.
Being able to choose how notifications are done is a great change as well. You can have your less important push notifications sent to a banner rather than to the non modal alert box. Very nice very good.
Apple is pushing iCloud, but I really am not very interested in that. I might be later, but right now... Meh.
The final new feature that I really like is that you no longer need to attach the phone to iTunes to update the software. This is a BIG help. However it kind of spooks me, because of my experience with updating my phone to iOS5.
During the install, my phone crashed, and I had to restore it back to factory preset, then I had to restore my settings and Apps. Then I had to put those apps back in to their folders. That took a long time and it sucked. Double sucked... If I were performing this update with out iTunes, would I be out of luck? With iTunes, I get a backup and I can restore the phone to somewhat of a state that it was in before. With out iTunes, will the phone just puke and then be dead? Something to think about... I know you are thinking, what about the backup that is made on iCloud. Think about it, what if I do a phone update somewhere I don't have access to a computer with iTunes for some time. Without the computer, I can't set the phone back to factory then do the restore of the phone. Will the phone just be toast until I can find some computer to plug it in? Or does the phone update by throwing everything in to memory... Sounds very inefficient, and lots of security problems come immediately to mind, but that never bothered Apple before...
All in all, good update, and I reccomend it for all of the iPhone 3GS users out there. With this update I think I can put off getting the next version of the iPhone until iPhone 5 comes out.