Residential meters are very insensitive to the power factor and therefore you would NOT save any money with these devices.
On the other hand they do reduce voltage noise and are also used to help X-10 signals jump phases.
I don't see anything illegal with using cap's since improving your power factor will help the utility and at the same time won't change how much your meter bills you for.
FYI: The only sure fire way to reduce your electric bill is to use less power.