logon/logoffs are also be written to the log tables (another thing you can monitor)
Cindy Clayton created a universe against the audit tables … More details here: Report on WEBI audit information
Here’s another good thread discussing the tables themselves: Audit Tables
As for performance, Yep, there’s will somewhat of a degredation, but worth it overall, best bet would be to pilot it turned on and judge for yourself.
Chris Pohl 
(BOB member since 2002-06-18)