I am using Telegram from over two years.

Pros:

- End to end encrypted secret chats

- Awesome bots and the bot platform

- Open protocol so we can implement third party clients. So is available on any new platform (Ubuntu phone, FirefoxOS, Linux, Command line, web,...).

- Recent improvements like end-to-end encrypted audio calling (Not sure if Whatsapp calls are encrypted)

- Many small features that make me happy like we can lock the app with password, customize interface, etc.

Cons:

- Not many users (On the contrary I like it as there is not much noise)

- No video calling

- There are some reports doubting their encryption