Free Forever challenge pricing experiment - feedback?

I'm in the process of launching https://dailystate.me, a motivational journaling app. It's in beta, but I'm already thinking hard about one of the most difficult aspects of launching: price vs adoption.

At the moment my goal is to get feedback, and make the app super useful. So free is the way to go. But I don't want people to sign up and check the app out for a minute or two. I need some serious usage.

So today I came up with a 30 day challenge:
If somebody journals for 30 days that person gets lifetime free.

I thought it's a good idea because:

• I want people to sign up who really test the app and provide feedback.
• if my app can't make you journal for 30 days, that's seriously good feedback.
• 30 days is about needed to create a journaling habit.
• Once the habit is there, I can ask the most important pricing question: 'How much would you pay for this?'
• Whether someone does the challenge or not, the app is free. But I might charge people later (with a good offer) who didn't make the 30 days and want to continue the using the app.

What do you guys think?

(And if you're up for the challenge 😉)
PS. I will definitely post the outcome of this experiment here!