This great little box serves a few purposes and, for me, has become a more or less indispensable part of my set-up. Principally, I use it to attenuate the output volume of a Fryette amplifier (100w, running in half-power mode) for playing at home, which also has the effect of allowing me to run the power amplifier at much higher levels than would normally be possible at home. I also have it permanently connected to my recording set-up via a TC Electronic interface.
Using it as an attenuator, I don't notice any colouration of tone and, in fact, using this device has really brought my amp to life. While it's possible to set my amplifier to levels low enough for home use, being able to do that while running the power amp at high levels has obvious tonal benefits.
By disconnecting the speaker connection, the Dummybox will attenuate 100% of the signal, so it's possible to record and practice late at night if you want to, either monitoring the signal from the computer or via the Dummybox's headphone output.
I find the pre-set, toggle-selectable attenuation levels are perfect for my purposes: lowest setting for evening, middle during the day. The highest level is perfect for live or rehearsal purposes.
As a recording device, I find the speaker emulation settings are perfectly good for rehearsing parts or making quick recordings of ideas. They need a lot of processing for the sort of polished sounds you'd want on a final recording, and personally I've never managed to get a sound I like for that purpose.
However, the unfiltered output is great for feeding the signal into software speaker emulation, e.g., impulse responses, which provides better sounds and much more flexibility.
In all, this thing does everything I need well. My only criticisms are the decision to put the connections on the same side as the controls (mine sits on top of my amp, so it means having cables trailing down the front, rather than the back, of my amp), and the need to unplug the speaker for silent operation (rather than using a switch). Minor complaints, really.