We've recently deployed SEP 12.1 on our Macs. Despite what Symantec claims (or at least they did 6 months ago) SEP 12.1 DOES NOT support 10.4. For the most part it's been a pretty quiet piece of unnecessariness -- some people initially claimed it was slowing their mac down, but I think this was due more to user-perception after getting something they did not want (and had probably read was bad for your computer performance given Symantecs reputation).
Once we got over the initial hump of installation and config nonsense, all works fairly smoothly. One note though, and I know this applies to several vendors: do not count on Symantec for support. Everything we've run into was met with either "that is designed operation" or "ummmmmm...." or a half-assed answer that got me part of the way to fixing the problem, but the rest was home-brewed solutions and detective work (my favorite kind).
I've written several articles about my trials and tribulations with Symantec over the past few months if you are interested: http://acdesigntech.wordpress.com/category/symantec-2/
Most of the stuff in those articles is not available at all, or only partially on Symantecs site.
