I own one. As far a cleaning your power, if your equipment is designed well, then it shouldn't make a difference. I have used the Monster HTS3500 on two different receivers. The first was the Sony STR-DE597. It is a pretty low quality receiver IMO, when I added in the monster, I could hear a 'cleaner' playback from music. I have since upgraded to a Onkyo TX-NR905, a much better receiver overall, and I couldn't tell any difference whatsoever.
If you have quality, properly designed gear then I wouldn't buy the Monster power center for its power filtering ability, but it does give you decent power protection which is all it does in my system at the moment.