Ranked in these QuestionsQuestion Ranking
Pro Provides security and privacy
Signal uses an advanced end to end encryption protocol that provides privacy for every message every time.
Pro Free and open source
Signal is free and open source software, enabling anyone to verify its security by auditing the code. It's the only private messenger that uses open source, peer-reviewed cryptographic protocols to keep your messages safe.
Pro SMS/MMS support
Signal can be your default SMS/MMS text messenger. Encryption is not possible when sending SMS to your contacts, however.
Pro Supports encrypted group chats
Pro Supports sharing of various different media types
Signal supports: emoji, pictures, videos, audio, contacts, any location and GIF.
Pro Message mirroring on Signal Desktop
Signal Desktop is a Chrome extension that allows you to mirror messages from your phone, including replying.
Pro Recommended by Edward Snowden
Edward Snowden uses this messenger and recommends it to everyone who is concerned about his privacy.
Pro You can easily view all media exchanged
You can easily view all media shared in the chat without scrolling back to when it was shared.
Pro Note to Self feature
Allows you to "send: messages to yourself and sync to desktop so you can use Signal as a kind of encrypted Pushbullet alternative
Pro Works everywhere in every country
Unlike most messenger apps, Signal works reliably in all countries by securely circumventing internet censorship.
Pro Annonymous Sender (Encrypted)
The sender credentials can be encrypted with the rest of the message, leaving only the recipient address readable by the Signal server. Even if Signal wanted they couldn't see who is talking with who by this mean.
Con Needs access to your phone number and contacts to work
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.
Con Unreliable notifications
Sometimes messages won't be received if the Signal app has been closed for a long time or hasn't been opened after booting the device.
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.