I think you need fairly full functionality in order to compete in the market, which is critical. (I understand that many of the current users just want some basic funtionality but i'm thinking about the future of the company not just my own requirements) At a minimum you need to be able to collect thoughts, and process action items, mark them done, schedule them, etc. There will be a larger market out there for "iPhone (or Touch) only" users that would not synch to a PC. Time to market is important as this space is filling up quickly and I think you're already late to the game - one of TR's competitors has already released a version of their app which appears to be fully functional stand alone
