On why jailbreak:
I've been an on-and-off jailbreaker since before the App Store existed, starting with the original iPhone.
First of all: I've never pirated an iOS app. Never have and never will. It's ok to do if you are doing it to evaluate software that doesn't have a demo/free version, but I don't even do it for that case.
iOS 5 is great, but there are a few things I'd like to tweak. For example, changing brightness is a pain with stock iOS and is something I do often; with a jailbreak there is a widget to toggle 20%/80% brightness from the notification screen (as well as a slider you can bring up). There's a bunch of other little things that I'd like to tweak, some of which may not even be possible with current jailbreak apps, but I'm a developer so I can try changing things myself. From what I understand MobileSubstrate works pretty well.
Should I have just gotten an Android phone instead? Maybe. I evaluated the Galaxy Nexus before getting an iPhone 4S, but ultimately I was unimpressed. Android has noticeable lag in all it's operations, even with ICS. The browser rendered things incorrectly often. Having the touch-screen Back/Home/App switcher buttons so close to where the keyboard pops up caused constant accidentally keypresses. I don't notice the 500ms audio lag, but I know it prevents certain types of music creation apps to be produced (I use music creation apps on my iPhone). Tons of little things. Galaxy Nexus's AMOLED screen was not as impressive as I thought it would be (especially if you lower the brightness below maximum). Maybe I would've felt different if I just bought the device and spent time using it. iOS simply has a faster UI, which I think is really important for the device I keep in my pocket. Maybe I'm wrong, and maybe Android's flexibility would make up for it's laggy UI. I'm probably going to get an ICS-running Android device at some point, because I know Java well and it would be cool to hack on it. My coworker has a Galaxy Nexus, and so far I am glad I got the 4S. He is always complaining about something not working.
Interestingly, I heard from one of the jailbreak developers that iOS is easier to hack on the Android. MobileSubstrate makes it easy to do ad-hoc modifications, whereas with Android you have to recompile things which can be slow, especially if you don't have beefy hardware.