Power is controversial, so there are no easy answers. A lot of the effects of a conditioner will depend on the quality of the power sections within your gear. Equipment with well-designed power sections will see less of an improvement from conditioning the power, because the power is being filtered within the component. Cheaper or inferior components may benefit from additional filtering outside of the gear.
But like the gear itself, not all power conditioners are created equal. They have different designs (series mode, MOV, etc.), and perform differently. How they perform will also depend on the quality of power coming out of your wall, and whether or not the power conditioner is prepared to correct for the deficiencies that your wall power presents.
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All disclaimers aside, in a home environment (as opposed to an apartment, where the power situation might be out of my control), I've found the effects of inexpensive power conditioners to be entirely negative on my mid-fi gear. A bigger improvement could be made by creating a dedicated circuit for audio.