Oh God, the PIN creation is so annoying. Whatever possessed them that that was a good idea? Note that even after creating the PIN the app doesn't quit nagging you, it 'helpfully' continues to remind you to re-type it so you don't forget.
Plus, it's just a terrible UX for a SMS client. I really wanted to like it, but it's just too obnoxious to use. I've certainly not recommended it, I've had better luck switching relatives to Linux mint (seriously, I am not exaggerating). Creating a group MMS is annoying, and there's a nag screen if you try to call someone from a message within the app. I understand that most of these problems are due to the secure nature of the app, but trying to use it as your full-time SMS client is just too much of a pain. I'll likely still keep it installed, as the 1 thing it does do is make it really easy to transfer files/send messages between devices (send messages to yourself, then download them using the client's for other OS's).