Signal is solid, though a few privacy advocates criticize the dependence on Google Play Services on Android phones. I also get the impression that development has slowed, with the developers (Open Whisper Systems) focusing on implementing the Signal Protocol in other clients (but correct me if I'm wrong).

There's also the newly open-sourced Wire - it has the same dependence on Google, but it supports video calling and some might prefer their UI.

Signal replaces your SMS app, which is good because it's perfectly seamless and there's no good reason not to use it, but bad because most conversations you have with it will be unencrypted because the other user doesn't have Signal installed. Unencrypted communications look pretty much the same as encrypted ones. The strategy is to eliminate inconvenience in order to increase takeup and challenge mass surveillance. But the tradeoff is that it's not such a great solution for an individual who really wants their comms to be secure.