HTTP Proxies used for Fighting Spyware: Feedback

[Two Bit <two.bit7 () gmail com>]

Hi there, long-time Nanog lurker network engineer with a (maybe off-topic)
question related to network architecture solutions to fight the
spyware/greyware problem. I was wondering if anyone might have any
experience deploying anti-spyware solutions which reside on HTTP Proxies.
Several products claim to be able to detect spyware on the wire such as ISS,
SonicWall, Fortinet, Astaro, BlueCoat. However, I am concerned about the
performance, especially since they have to use an AntiVirus product on the
back-end (heavy processing). Curious what the user experience might be, how
effective any of these solutions are in really catching spyware, and any
other operational experiences from engineers employing any of these
solutions out in the field (not from vendors, please) that may help narrow
down the choices. Thanks for any input.