Recs.
Updated
SpecsUpdate
Pros
Pro Recommended by Edward Snowden
Edward Snowden uses this messenger and recommends it to everyone who is concerned about his privacy.
Cons
Con Single device
Signal can only be registered to one mobile device at a time. But you can link Signal to Signal Desktop.
Con Servers hosted in the US
A security risk due to National Security letters, which require giving up data to the US state, and making it illegal to disclose that.
Recommendations
Comments
Flagged Pros + Cons
Out of Date Pros + Cons
Pro SMS/MMS support
Signal can be your default SMS/MMS text messenger. Encryption is not possible when sending SMS to your contacts, however.
Con Cannot work w\o Google services
Signal relies on Google services for push notifications so although it is encrypted and open source it needs proprietary component to work. That means you can't use Signal on some custom Android ROM (preferably rooted for full control) without Google apps installed.
Con If SMS/MMS imported, other apps can't read SMS
For those using Join, Pushbullet, or similar apps to send and receive SMS/MMS from web or desktop, putting your SMS in Signal somehow breaks this functionally. The dev is aware but plans to remedy the situation have not been named a priority.
Con Signal requires Google Apps, which is privacy invasive
You first need to install closed source, privacy invasive Google tracking system into your phone by way of Google Apps, which gets access to everything that happens on your phone. Only after installing this Google Apps, are you able to use Signal.