A few things must be considered before using this software. (The software itself is not affected.)

1. Local law might not allow monitoring users at all.
2. Local law might allow monitoring at some kind of degree (where ttyrpld goes pretty deep)
3. If monitoring is allowed, you may need to notice your users about this
4. Different kinds of environments have different monitoring restrictions, i.e. home, within companies, educational and/or governmental institutions, a company's shell services
5. Monitoring is usually limited (maybe not by law, but through contracts) if the users are allowed to do private things
6. Monitoring even private data might be granted (not necessarily!) if you (as sysadmin) can guarantee that any data captured whatsoever is only ever used in cases of abuse or suspected abuse.

Please consult a lawyer if you are uncertain about your status.