How did it compare against the built-in de-dupe tools in Windows Server 2008/2008R2 ?
I think you're comparing apples with oranges. You're referring to filesystem de-duplication I presume which is a something of a different application albeit the technology is similar-ish in concept. Not played with 2008 particularly so couldn't really comment. I don't spend a lot of time tooling around with servers any more, and if I do it's usually something a little more heavy-duty than Windows.
The Riverbed stuff works by checksumming chunks of data as it traverses a WAN link and caching it. Next time a recognised chunk hits the sender it discards the payload and sends a token to the receiver which serves up the data to the destination host. You also have the option of cutting a cert to allow SSL decryption; man-in-the-middle style; and cache that content too.
Chatty protocols see a substantial improvement but stuff that's already heavily optimised (eg. ICA traffic) will show very little.
PPRuNe is not the place for technical debate
If it's in the computing forum I don't see why not.