Pros
Technical proficiency is really valued at Appear, you get to work with colleagues who value clean code, and my time there made me a lot better programmer. The management was very understanding with my professional wishes and feedback. It was a real constructive discussion throughout the 3 years I worked there. I am now trying to be a better professional thanks to what I've learned working at Appear, both on the technology level and human level. Technically, the work is challenging, because the product is complex. So if you're looking for software challenges related to enterprise mobile apps, you won't be disappointed! I've contributed in building some mobile apps whose UX I'm really proud of, and this is not a given when your clients are the transport industry. Positive atmosphere there, I really felt that there was a synergy between the team members, although I've had my share of tough days (like in most jobs), due to the complexity of development. After my time at Appear, I didn't go for a position in yet another software company, because I knew it would be hard to top this.
Cons
I thing the challenge when working at Appear is not due to the company itself, but the business sector. When your business is making tools to ease mobile app development, you compete with super strong players like Google (AngularJS, Firebase...). So it can be difficult to work in the context of a very competitive business sector where it's difficult to win customers.