The word "Android" is just the opposite of the word "Stringent". Means, there are lot of different navigations that are available. It all depends on what you prefer. I basically prefer LauncherPro, but there are few people who like SPB Shell, Sense UI and if you get into custom rom like cyanogenmod, there are TOOO many small features that will make you beg for android. Ex: I guess you already knew about the classic "Notification Bar" that had been recently copied in iOS and Symbian Belle. What this notification bar basically does is (apart from notifying), allows you to toggle between wifi on/off, gps on/off, bluetooth on/off, silent/vibration, auto rotation (available in LG, Sammy and New Sense UI). But the Cyanogenmod lets you go beyond it. It allows you to choose what toggles the notification bar can have and apart from the above it also adds up display brightness, music, 2g/3g switching etc. And that's not all, it allows you to get into that particular settings page directly, if you long press the icon.
What I've explained above is just a small percent of what cyanogen a.k.a android o.s can do. Some of the other features that cyanogen packs are bottom slide notification bar (with big displays, it's harder to pull out the notification bar from the top, with cyanogen, you can drag it from bottom of the screen. No unlock screen, blazing fast UI and much much more that I can't remember...
Edit: Cyanogenmod is currently being cooked for SGS II. But I'm more than sure that you'll be happy with SGS II + Launcherpro