Here's what I've had to say about power conditioners/expensive surge suppressors in the past, it’s still generally good advice.  These days at every electronics superstore in North America you can find Monster Cable peddling surge suppressors that may make other bold performance claims like line filtering.  Peripherals like surge suppressors, extended warranties and interconnect cables are usually marked up considerably and how the big box stores make money off the sale of very low priced hardware.  But do you need surge suppressors or power conditioners?

The quick advice on surge suppression is to that it is a very real factor for electronic equipment.  Light strikes near your house and it can send high current through your wiring and can damage equipment.  Although relatively rare it can happen.  Surge suppression has a rating system in place so a suppressors effectiveness can be measured.  Look for their rating in joules; the better they are at shunting a surge to ground the higher the joule rating.  If there is no such rating, it’s not a surge suppressor and doesn’t conform to IEEE 587 standards.  If you own your house, consider surge suppression on your fuse box to protect everything in your house at once.

As for many of the other claims made of many of these devices, such as line conditioning, voltage or ground stabilizer and noise filter, you must be more than a little skeptical.  None of these jobs have a standard (like surge suppression) or a universal rating to measure effectiveness.  You must basically trust the manufacturer and in the case of Monster Cables they’ve proven to be quite untrustworthy.

I tend to believe much of the noise about power conditioning is fiction.  Consider exactly what job you're looking for and why you need it.  Voltage stability can be a problem if you’re suffering frequent brown outs and you have gear that is very sensitive to fluctuations.  The owners manual can usually let you know.  But most moderately priced electronics gear have power supplies capable of shrugging off minor surges and voltage irregularities.
 
Keeping your AC pure for better quality audio is the kind of thinking that borders scam and usually is.  If your sound is being affected by dirty power and not filtered out by a decent quality power supply, it’s usually in extreme cases only and temporary.  Turn your vacuum cleaner on while watching a movie and see what I mean.  If the vacuum cleaner is operating in close proximity to your HT hardware the motor’s oscillation will reflect back through the AC line and will likely cause disturbances in your TV’s picture and might even be heard through your audio system.  But this is an obvious problem.  I get annoyed at the idea that there are minor irregularities on my AC line that I don’t even notice and an expensive power conditioner is going to filter it out and unlock never before heard performance in my audio system.