We've had a 400 up for 3 months now, very nice box. It's doing pretty
good with handling false positives, and the updates are very frequent.
One of the nice things for us is the ability to do per domain / per user
quarantines for clients that want that ability. It also has an
'Exchange Accelerator' to tie into LDAP on exchange servers.
We've not had any serious through put issues so far, but only pushing
500K mails a day so far.