Around 6 weeks ago I switched from using the iphone 3g to an android nexus one. Overall it has been a great move for me, particularly because it helps me to follow Gtd principles with greater ease.
First of all, as a gmail user, the android allows me to archive messages that aren't actionable. On the iphone there's the trash button but no archive so it wasn't any use to me for emptying my inbox. Gmail on the android is snappy and can compose messages of when offline, unlike the gmail iphone webapp.
The next killer feature is the "note to self" voice action. It is the quickest capture device i've ever used especially when a hotkey is assigned such as a long press on the search button.
Calendar snooze application is another really great feature and I often use it as a tickler file.