Best Firewall Appliance For an ISP?

Home users can simply download a free copy of ZoneAlarm, but a reader from South Africa who runs security for an Internet Service Provider (ISP) wrote to the Netsecurity Forums looking for advice on what firewall appliance to use:

Can some one tell me what is the best Firewall for an ISP? We have installed a Sonicwall Pro 20/40 but it falls over now and again. [Does anybody] know [about] the Astaro Firewall or the AstroFlowGuard?

If you have any experience or knowledge with firewall devices for medium to large networks, perhaps you can share some information. To read this post, and the replies to it, visit the thread in the Netsecurity Forums.