As a Day One user I thought I'd have a play with this app.
Pros: Nice clean interface. Timeline is a neat idea (but it only spans one day). Entering information is easy and the controls you're offered are easy to access. Has a useful search feature. You can tag entries and group entries by type.
Cons: Window sizing issues (i.e. resize window, content stays fixed). Very prescriptive - about the only thing you can configure is your name, so if you don't like the layout, theme, icons etc., to bad. Can't add audio clips. No iOS version/sync (although I understand that's coming). Price.
The biggest sticking point for me would be the price. I appreciate devs want to be rewarded for their efforts but compared with what other diary apps offer, this just seems rather expensive. At this price it needs some killer feature to differentiate itself, but I'm not seeing it as yet.