Ovnilab review: http://www.ovnilab.com/reviews/darkglass.shtml
Concluding paragraph below. (I think it is fair to say that Doug at Darkglass has done a really good job.)
[quote]Compared to the MXR M87, the Darkglass is larger, but better constructed; and the M87 has brighter highs, while the DG has a fatter warmer sound. The Empress is the same size and quality; it has better highs and clarity, while the DG has fuller lows. The Keeley Bassist is smaller and more neutral in tone, while the DG has MUCH better metering. I actually think I'd recommend the Darkglass most to someone who was considering a tube compressor for its warmth or "vintage sound", but not wanting to use an uncommon power supply. It's quite similar to the Smoothie with the tone tilted a bit to the bass side, though again the DG has better metering. The Origin Cali76TX has a better combination of deep lows and clear highs, and I just love the Cali's action, but of course it is much bigger, and lacks the blend function. [/quote]