As many others have emphasized, the quality of power into your home is important. I'd say equally (if not more so) important are the other devices in your house. Many audio components have some aspects of what a power conditioner does, built right into them. Other devices in your home can create significant electrical noise (think major appliances).
A power conditioner may offer the following features:
- Surge protection (MOV, avalanche, etc...)
- Power regeneration (very inefficient btw, if you care about your electrical bill)
- AVR (automatic voltage regulation)
- Voltage monitoring (may shut off power if voltage is too high/low)
- Isolation transformer (like in RGPC products, or an APC H15)
- RFI/EMI filtering, often with CMC's (common mode chokes)
- May provide balanced power (BPT, Equi=Tech)
I'm sure there's more, but I think that covers most power conditioners. I haven't seen much data about how effective they are in real world environments, or even controlled blind tests to show audible improvements. It's easy to create conditions to verify that the conditioner does what it advertises (i.e. feed it 100V, and it boosts it to 120V, or verify the surge protection meets the advertised specs), but knowing whether these conditions occur in your home or would even have any real noticeable effect on your equipment is a totally different story (and hard for anyone to say, because different people have such different equipment).