My take on ISA Server v IPCop so far? If you've got the money, ISA Server 2006 wins. IPCop is a great product, but I like ISA Server 2006 is better. It is very easy to use and has a lot of features. I had a helluva time getting IPCop installed due to driver problems where Windows immediately picked everything up (not a Windows v Linux discussion however).
The downside to ISA Server 2006 is that is expensive whereas IPCop is free. ISA Server 2006 Standard is $1,500 per proc and ISA Server 2006 Standard is $6,000 per proc. And yes, cals are required- servers are $1,000 a pop and clients are $22 a user. External cals "vary" in price.
I'll update this thread as I learn more about ISA Server to compare and contract the two products. I got a network with five servers setup and running in about an hour using ISA Server 2006 (IPCop is about the same if it has the drivers you need).
Tom Hundley
Elegant Software Solutions