Some people don't like the GUI interface but if that doesn't bother you it is a great tool for astronomical image processing. The documentation is a bit sparse but there are a bunch of decent free video tutorials (e.g.
http://www.harrysastroshed.com/pixinsighthome.html) and one paid tutorial under development (
http://www.ip4ap.com/pixinsight.htm)
It's definitely my preferred processing environment.
Cheers,
Rick.