I owned a Stellar Powerplant P3 for a few years, and recently upgraded to the P12. I really like it. I thought the P3 made an improvement, but the P12 is even better and makes my system sound consistently good. I only use it on my front-end components (primarily because my front-end components are located a fair distance from my monoblock amps).
PS Audio PowerPlant 12 Review (AC Regenerator)
This is interesting. The testing concludes that the filtering inside of the actual electronics device is what matters and that this PS Audio product actually adds its own noise. Some irony.