Pros: True GTD, location-aware, syncs with Mac
Cons: Occational crashes
David Allen's "Getting Things Done" (GTD) methodology for productivity is the best I have found. The Omni Group's OmniFocus for iPhone is the best mobile application for managing GTD on the go.
I purchased OmniFocus for the Mac when it was first available, and the iPhone version closed the loop. I now have a synchronized version of my next actions by context and project wherever I am. Having the applications everywhere I am tends to underline the times when I wander away from the methodology and back to the "manage by Inbox" that I used for years.
The location-aware aspects of OmniFocus allow you to set contexts for particular locations, then allow the phone to tell you which next actions you can do where you are. This is very useful and a shortcut for accessing the appropriate contexts.
Syncing is the most frequent cause of crashes, especially if I attempt to add or change items while it's syncing.
However, if you are a hard-core GTD user, this is your iPhone application.