Is the Galaxy S II stable?
As an Android user for about 2 years (and an iPhone user for 2 years before that), I feel that Android is very stable.

BUT you have to remember, that with the ability to load widgets, and background applications - you can certainly bog down your phone and decrease its performance.

In Apple's walled garden this isn't a problem, but you also don't have the freedom and choice.

As a modern computer user, I want the choice to slow down my phone if I want :)