I've been using Day One for years. As another person said, it's full of features — that I mostly don't use.
So I'm super interested in trying Journal for its simplicity, and iOS integration. Similar to how I use Calendar, not Fantastical. Fantastical is gorgeous, but also a clunky hot mess for someone who just wants to glimpse at an event now and then. Similar to using Notes, versus Evernote.
I journal weekly, usually when I have insights. And similar to Dan, I like to journal during trips (when I take tons of photos) and other moments. So I want to see how Journal might pull those things in automatically, or organize them automatically, etc.
One feature about Day One that I love is tying a song to an entry. If Journal integrates songs — especially in a more dynamic and present way than Day One, where it's hidden in a menu — I'd love Journal for it.
The only little feature I might miss from Day One is that I use the internal app links to cross-reference journals. It took Notes 16 years to get that feature, who knows when Journal might get it, if ever…
Also — and this is sort of odd — but I really like Journal's suggestions so far. I've never used this in any diary app. But I'm inclined to answer the ones Journal is prompting me with; I automatically have an answer or insight into what it's asking, so why not?
Interesting that it appears Apple made it a purple app, changed from the keynote's white app. It joins Podcast as the only other purple app? I might like the light/white version more. EDIT: I see it only goes purple in Dark Mode, instead of black. The purple is indeed nice for Dark Mode.
EDIT 2:
Just got it installed. Since this is my actual phone, it pulls in data from other places on the phone, like Music and Photos.
Yep, the music integration is SUPERIOR to Day One. With Day One, it's just a metadata attachment; in Journal, it's a minimal player with a text where you could actually reflect on the songs. Music — especially when first discovered — can serve as its own moment in my life, with its own thoughts, so this is pretty nifty. And I often discover super meaningful songs, then forget them in a playlist. Journal immediately pulled out a group of songs I would never consciously remember listening to several days ago.
As for Photos, it's pulling up some interesting moments from the past. Seems like it's targeting time periods that have multiple photos taken within it, constituting an "event." I could certainly write about some of these, though they're pretty far in the past. When it comes to photo-moments, I might describe Journal as the "text companion" for the visual moments that the Photos library generates. I'll have to see how this handles with new moments/photos in the future and in real-time. That'll be more useful for me.
So the way it's looking, the way I might contrast Journal with Day One, is that Journal is a "text companion" for multiple other areas of iOS activity. Music. Photos. Someone else mentioned it pulled in a hike.
Imagine going on a hike where something interesting happens and you take photos, or the perfect song comes up and makes you think of something. Journal would 'automate reflection' on those types of things it seems.