I've been using Linux (CentOS5) on gigabit firewalls, for thousands of users. No problems.

Just make sure ip_conntrack_max is big enough, so you don't run out of connections.

There are other things to tune to optimize the performance, but it's certainly doable with linux+iptables.

-- Pasi

Would you provide us information about how the firewalls are configured, the hardware, the total bandwidth passing the firewall in each direction and the connections in the busiest hours.

regards