I've been building apps with the Ionic Framework for a year now, and I really like it. A big improvement over jQuery Mobile, especially if you only need Android, iOS and Windows phone apps. And now with Ionic Creator, I can create a mobile app prototype in about 15 minutes, check it into Git and start using other Ionic tools to complete the development.
Ionic Creator is free for a single project and the signup is easy. I pretty much just use it to get started, flesh out a project. Then I check it into Git. I could see Agencies or teams wanting to manage multiple projects at once which would require a paid account.
It helps if you have your app goals, features and menu navigation at least in mind when you start. The tool is so easy to use that it does almost eliminate the wireframe step for simple apps -- the Creator is all drag / drop and the end product is a functioning AngularJS mobile app.
I really like how Ionic is really just an AngularJS app, nothing special at the most basic level. The Ionic libraries take things to the next level. If you can hack AngularJS, you can create great mobile and web apps with the Ionic Framework and Ionic Creator.