I'm an RTM user as well
I'll start carefully here by sharing that as a David Allen staff member, I'm not endorsing this app! (The app world changes too rapidly for us to endorse individual apps in the marketplace across the mobile platforms.) But I can share my personal experience.
I keep my trusted system in Outlook 2010 and wanted to synch with my Android. I also tried and failed with a number of apps due to the lack of ability to synch the categories / category view.
I'm using the Pro Version of Remember the Milk (RTM) and it works really nicely. While I'm synching to Android, I've heard from others that this works well to the iPhone as well - though I can't verify that personally.
I'd be curious to hear more on this thread about what has worked for others??
Coach & Presenter
David Allen Company