I currently use "Day One". Not free, but it sync's up between my iPhone, iPad and my MBP. Rather than detail what is written on their site, here is their site:
Day One | A simple Journal for iPhone, iPad and Mac App Store
Like I said, it is not free, but it works well and the sync is flawless. My thing was that it was simple to set up, use and sync between devices. I am using it for short snippets of text (like a personal twitter) though you can do full on journaling with it and add photos and more.