It works with a pin, has unlimited messaging, no restriction on character limits etc. You can use this as a stand alone app or as your sms text app as well.
It shows current status, message delivered indicator, message read indicator and more.
A very nice start to what looks to be a very promising app for all smart phones.
Give it a try, you will love it.
(disclaimer- I have nothing to do with this app, just sharing a good find)