Hey, Ed.
It adds a small chunk of memory to track a table w/ user and session details in it, and this table must be updated during login/logout processing. In a cluster it can result in additional cluster messaging going between the nodes.
I mean, it's not going to make your system crawl, but it is going to add some overhead.
Personally I've never found live users to have much value. It typically only knows about users on the current node (not cluster wide) so unless you can control ability to connect to each node, you can't get a good view of users anyway.
Even if it were working, the only thing it offers is the ability to force-kill their session. You can't message them, you can't see what they're doing, ...
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