My friend is running one and he thinks it works fairly well. However, whenever I see it, I much prefer the performance of my current Euroreef or my previous AquaC EV-180, Bullet 2, and MTC HSA250 beckett skimmer to the performance of his ETSS. The Reef Devil's reaction chamber and riser are both relatively small compared to the other mentioned skimmers. For a little more money, it is my opinion you can get better performing skimmer then the Reef Devil.
Also, when I owned a full-size model of ETSS (800), one of my dislikes was how sensitve it was to water chemistry and water height. Although it's true that adding foods or sticking your hand in the water will affect all skimmers, I found my ETSS particularly sensitive to any changes, sometimes not skimming for days because of some little change. And every now and then, it would produce excessively wet foam. My Euro-reef, on the other hand, is a lot more reliable and consisetent.
Just my opinion, of course. Hope it helps.