I used this years ago very extensively. It was a fork of Smoothwall a UK firewall.
I first started using this as Smoothwall on dialup and then as IPCop on ISDN.
One of the features was the transparent proxy caching. which helped with updates and web browsing when an office full of computers was connected to a slow Internet.
At the time it had a very active team of developers who kept the security up to date. We stopped using it after we discovered DDWRT for the Linksys WRT54G router. OK not as powerful as a PC or IPCop but right for what we needed once we had broadband.
Even DDWRT is not so important now that even a basic TP-Link router has pretty good features.
I do know a company who bought a professional router with the latest IPCop and use it currently but I would no longer recommend this for new installations.
I really hope that they won't discontinue the project because I am very impressed about this massive firewall solution. Very flexible, reliable, loophole-proof and imho a must-have for security aficionados.
I've been using IPCop since 2006. It is fantastic, very customisable and rock solid. I run it on an Atom D410 with 2GB memory and an SSD, in a fanless VESA mount case. I use the Snort and Guardian add-ons, among others. I also run some other stuff on the same machine, like Minecraft server and AssaultCube server. I wrote my own monitoring tool for the hardware using gnuplot, html and bash. I used the TCC to build rkhunter on the machine.
I initially used it with PPPoE with a Kyocera modem and burst technology (I am in ZA) on Red. Now I have a Huawei 4G LTE router that connects to Red via dhcp. On Green I have an SMC EzConnect switch that provides safe, filtered and snorted internet to my family.
The community is helpful, in the ipcop-user mailing list as well as the Sourceforge forum. I have had a plugin-developer make a custom version of a plugin (Snort) for me for Eth over USB.
I am actually running 2.1.9 but it is not configured here. I installed 2.1.5 and the rest is done with updates, which work seamlessly. It has a vibrant following in Germany. I hope the project doesn't die. Kernel and other software is getting a bit long in the tooth.