I've used it and that's all the proof that I needed.
Sorry. I wasn't trying to sell you a copy. I was curious why you have the opinion you do. User interface? Features? Security?
I used to use a LOT of ISA Server because it was basically "free" to my clients. It has a ton of features, but I found it very difficult to learn. I basically took a year reading books on it, taking tests, and trying to figure it all out. I can't say that I was 100% successful.
It's not free anymore (well, its Forefront equivalent), so I doubt I'll be doing any installs except on my own servers.