We have one (Redsea 25) in a XL300 (36" wide tank, 65 gall display) along with the regular powerheads. I love this thing. I find it gives me better flow without the jet-engine turbo effect. Our powerheads are probably too powerful, but the gyre creates a feeling like a wall of water is moving through the tank as opposed to a blast from a hose. It was easy to find places where even the mushroom corals weren't annoyed.
I wish I had just gotten like 3 of these instead of the powerheads we have. The low setting on this is nice and LOW and we pointed it at the mushrooms that get covered in sand by our gobies. It is able to gently whisk the sand off without ruffling their skirts.