I give them, or anyone, credit for trying to create a secure messenger. It is not easy. However, I just wish they would release the source code to their clients and server. They have not. That would go a long way.
Because the designers have their heads firmly planted where the sun doesn't shine, for one.
WhatsApp thrives because in many places, SMS costs are prohibitive (so TextSecure is not an option). In addition, it requires no registration and doesn't rely on external services (so ChatSecure is also out of the question).
It's just that I heard some concerns about key exchange (that triple Diffie-Hellman exchange) not having a formal security proof, although I'm completely incompetent to evaluate whenever those were valid concerns or just some chatter.
I have no idea whenever and how broken IGE is. I heard, nobody even cared to evaluate that. Boils down to "no formal proofs (but likely to be broken)".
At the same time, I heard the concern there are no security proofs on the key exchange and it may have issues. Since as a commoner I can't evaluate it any further than this, so boils down to "no formal proofs (although hoped to be fine)", too.
Those are surely different cases. I'm just concerned over what I use (TextSecure), though.