Hey Russ,
I run a Prowler, and found that if you agitate a carpet well prior to extracting then the built in filter got clogged up very quickly due to all the 'fluff' that's produced.
I got my hands on a 'Linthog' and the difference was amazing. I only ever have to clean the built in filter once a week or so now due to the 'Hog' now collecting all the crap.
I wouldnt run the Prowler without one now.
Cheers Daren.