Telegram should be considered insecure. See this post for a nice summary: http://security.stackexchange.com/a/49802
Their secret chats specifically seem like they would be entirely secure.
I do like that Telegram launched with an API, but ya, it sounds like security isn't something that can be trusted. Thanks for the informative link.
But in any case, you seem to have missed this bit: "The protocol they invented is flawed. Here is a nice blog post explaining why."