Don't trust it until full source disclosure. In a case of open source, it just means - go to the repository and read the code. If you have doubt in somebody's honesty you either don't trust them, or check them. Why am i so paranoid? Go to your settings, open them, and see HOW MUCH EXACTLY permissions (open sourced) telegram needs to work. You have no clue which data sends yours app, no idea how this data is processed by their service, and there is no way to decrypt your traffic and analyse whats actually been sent. I don't know what game they are playing, but lets summarize:

This is common known (Pavel mention this by him self), that all data from his previous project is accessible to Russian government without an order. Very suspicious move, and i have no justification or reason for that. Official client with source code that 'will be released eventually' (read: never). So, by the time you read this, all your data may be synced to FBI servers. Copy ALL the data, and then terminate their session. Then they just log in, and remove message 'new log in detected'. You just have to be on their territory physically. I've reported major bug months ago - any government surveillance service is authorized to receive your messages without sending them to your device. Great, so you do have thousands of the bitcoins, and you want somebody compete with you.

And then, OFFICIAL telegram sources promoting CLOSED source client, that way ahead of alternatives, and tell they doing it for competition. OK, said the audience, we skeptical about it, but it does make sense, so lets give it a try. You might read protocol specs and client sources, but no the servers, its our commercial secret. Once they told us that this platform is secure.

Telegram x for ios