"will I actually notice a difference in the picture or audio quality when using a power conditioner?"
With tongue in cheek I'll answer that you will, only after a lightning strike on a nearby power pole. These items are only additional line and surge protection. I install one per device in the house and the plan here is that with all the surge protectors installed the total number of joules that can be absorbed due to an out of bounds electrical spike might do no damage.
I just ordered a new TV from The Good Guys and the salesman was trying to get me to buy an $80 Monster Power Conditioner/Surge Protector and some $40 Monster component cables to hook up my DVD player but I told him I was gonna research it some before I spend more money on these accessories. Personally I am very skeptical that the power conditioner will do anything noticable to the picture or the sound. I'm not even gonna have the audio hooked up to a stereo, I'm just using the TV's built in speakers. Is this guy just trying to get me to spend more money or will I actually notice a difference in the picture or audio quality when using a power conditioner? I'd like to do a side by side comparrison with and without the conditioner but unfortunately the Good Guys won't return opened accesories. What a shock! Has anyone tried this?