Compare Xymon and Nagios

list Tinh Do
Wed, 1 Dec 2010 12:39:47 -0600
Message-Id: <user-a5b1799ed3d0@xymon.invalid>

some groups in my company start to use Nagios.
Has anyone used it.. 
Can anyone give the feedback regarding the Nagios. Pro or Con are welcome.
I need that information to defend my choice of Xymon.

Thanks