Advice: don't.

I switched from iPhone to Android, and while there are some advantages (mainly: I'm not on AT&T), overall it's not nearly as nice an experience.

Top complaints: poor battery life (I'm using a Samsung Galaxy S, but I think iPhone4 beats all Androids), inaccuracy of keyboard, and overall lack of fit and finish - eg. sometimes the phone will freeze for a few seconds and then come back to life.