I do contract beta testing and 99% of the apps I test are just "work" for me. But every once in a while I come across one I really like and actively cotinue to use (or play, in the case of a game).
I've never tested or even seen a "productivity" app that I've found really helpful, until I tested LifeTopix.
LifeTopix provides components for all the areas that people need to enhance their productivity (tasks, projects, events, shopping, finances, health, travel, etc.), and ties them together so data only has to be entered once. Plus, it integrates with the native calendar and contacts applications to carry this idea of non-duplication of data to the fullest. I have really found it helpful.
I tested it on both the iPhone and the iPad. Initially, I docked them pretty heavily for not having orientation support on the iPad, but that's coming by the end of this month (I've actually seen it in the latest beta, so I know it's really coming), so I cut them slack on that.
Anyway, I think it's worth a look.