I own the Keymaster and can't imagine living without it now. It lets me process my guitar effects in parrallel which is extremely benificial to my guitar tone. Not only does it excel in that duty, it also works amazing as a way to re-amp other sources such as microphones to take advantage of some of my nice analog pedals for amazing vocal effects.
The Pigtronix Keymaster True Bypass FX Loop is an extremely high-headroom solution to a number of problems. It can act as a DI or as a reamp device, and allows you to route just about any type of gear through any other type of gear - all with parallel processing! I personally love it for reamping, and running vocals and other tracks through guitar pedals.
The Utility pedal of them all
This pedal was made by musicians. Whatever instrument you are playing on whatever level, it is a must. A/B/Y box, using two amps, mixing two channels, mic preamp, effect switcher, DI box, whatever you want. You can buy half dozen boxes at the price of Keymaster each, and you won't get more. None of my pedalboard/recording setups work without it. Even if used as mic preamp, it provides excellent headroom. The ability to switch loops in parallel is great for running two preamps. A must for anyone.