I came over from the HTC Hero to the 4S and the phone itself is an amazing upgrade as hardware goes. The speed and smoothness of the 4S is leaps and bounds ahead of the Hero. With that said, there are some really simple things that I think Apple really missed the ball on.
I didn't realize how much I used the dedicated back button until it was gone. The same applies to widgets. I really miss some of mine being right on the home screen.
The App Store for Apple is controlled by Apple, not users, so 1. you have pay for FAR MORE apps than you did with Android and 2. there are few fewer actual utility apps that allow you to adjust, modify or control settings on your phone.
For example, with the Hero you could set up silent profile, vibrate profile and a loud profile. With the iPhone you have two choices- silent or loud and the settings you can adjust are minimal on either. I REALLY miss using apps like Missed Reminder (custom notifications for missed calls, texts emails etc) and I miss SMS Alert which allowed me to set custom virbration settings for texts, emails etc. No such things exist on the iPhone or the app store unless you are jailbroken.
If you plan to jailbreak, you might wait a month or two and see if the 4S is successfully broken and all the Cydia (jailbroken app store) apps are fully functional. If you're OK with only 8gb of space (for people that don't store lots of music or movies 8gb is more than enough) the iPhone 4 is a great choice and already IS successfully jailbroken.
Instead of HTC sync, you'll have to use iTunes to sync your phone and or transfer pics back and forth unless you use iCloud, which I do not. Too buggy for me.
You are limited to the type an amount of *themes* you can use and there are several icons that you cannot delete from the iPhone (newstand and some others I think) but that is just a minor annoyance for me.
Anyway- Yes, the iPhone is a great upgrade but the above are some things that I wish I would have known before I bought. I would have either gone with the iPhone 4 or maybe even stayed with some other Android.
Rick