How secure is Kerberos?

Kerberos is still used widely in Windows server and clients are included in all major OS's.

I would have to speculate to answer this really. All I can say is that, for it to have survived so long, it can't be all bad. It is, of course, limited mainly to private networks since both the server and the clients all have to trust the Kerberos server. This makes it unsuited to web-based developments.

Sorry, not a Kerberos expert so I can't really help here. All I can say is that a quick Google search brought up some promising articles.