I have it on my Mac, and it is indeed very good. But I decided against using it for two reasons:
1. It's hard to get your data out of. You're tied to writing and viewing in the app only.
2. I have an Android phone, not an iPhone or iPad so I can't view or edit my journal on another device.
I've decided to use Evernote. It's not as nice as DoyOne because my Journal is merely a different notebook within my others. But I can see and edit it on Android (or online) and I can get it out again as pdf or html files.
If, however, you're sticking to iOS then DayOne is good. Store it's data file on Dropbox and you can edit on several devices.