I have a Hog1x and it works really well. It did take a few months to really get a strong algae to take hold but now I get the most amazing algae growth.
One other benefit of the ones that go under the waterline is that I get a lot of "bugs" (copepods and I'm sure other fauna) as well as a snail factory. I grew 3 trochus snails from tiny specs to 3/8" in about a month and they kept the glass clean to!! Best part is I don't even need to pay these fools