The EFF has just wrapped up a review of various common (and not so common) IM systems and how they rate for security and privacy. Sadly a number fail when it comes to points like "perfect forward secrecy" (if your key is lost, it doesn't disclose all previous messages) or end-to-end encryption.

Glad to see the TextSecure gets full marks. Works pretty well as a secure SMS replacement system (no SMS fees!) but also supports unencrypted chat via SMS: https://play.google.com/store/apps/details?id=org.thoughtcrime.securesms