Yeah, I'd like to know too? I just bought a Redeye about a month or so ago, but I haven't gotten a chance to use it yet, since I still don't have an amp to reamp with.
It's not a great DI for passive guitars. You'll lose a bit of tone. Probably indistinguishable to most, but I always use my Millennia's on board DI these days because it retains more fidelity. I hate loss at the impedance conversion stage so the Red Eye isn't really an option anymore.
If you use mainly actives you can get away with it. It's almost indistinguishable. Almost.