rsync to completely mirror an entire machine

[Christer Edwards]

I do apologize if this has been discussed previously.  Looking thru
some of the archives I wasn't able to clearly answer my question, only
find bits & pieces.

Situation:
I have a web & mail server (production) and a backup machine.  The two
machines are near identical as far as hardware & software
installation.  I use the backup machine as a mirror of the main.  If
for any reason mail functionality dies I can redirect to the mirror,
etc.

My question is can I use rsync to safely mirror the entire drive? 
Currently I have just mirrored /var/www/ & /var/mail/.  I'm sure there
are other files that may be regularly updates (ie; logs) and I want to
make sure the backup-mirror is completely up to date.

Is it safe to just rsync [remote]:/ [local];/ ?  Would the /dev or
other folders cause issues with this?  Would it be safer to implement
a more detailed rsync script excluding certain areas?

I do appreciate the help.