Any body implemented workflow with apache Sling before? How easily to integrate 3rd party workflow engine for example activiti with Apache Sling?

I'm not sure activiti but when you are able to package it within an OSGi bundle, it ought to be possible for doing things in Sling.

The JCR and Sling observation systems are extremely useful in integrating with workflow systems, as they possibly can call back to your code when submissions are modified within the JCR repository, in an exceedingly decoupled event-based way.