I use ISA Server 2004 and planning on using ISA Server 2006. If you use a Windows or even use any Windows Applications I would say it's a no brainer to have an ISA box somewhere on your network even if it's not your front end firewall, it's very easy to setup for VPN and also web caching. It also makes a firewall in general. I know most people would laugh at using a Microsoft solution for security but I would have to say that ISA 2004 and up is an exception it really is a pretty good secure product.