Sound, sound, the sound. In addition to being full and rich sounding, I can hear the audio a full octave deeper on the low end with the Reddi than I have been able to with other DI boxes (like the Avalon U5) or the line ins on my preamps. That's amazing. It just sounds huge. The tube/transformer coloration makes things sit better in a mix than a more clinical DI would.
It might not have enough gain for some sources, for something like a passive bass you might want to follow it with a preamp for more gain.
Price is a little expensive for the hobbyist user.