Here's a link to more info:
http://www.tunze.com/usa/en-gb/dept_290.html
Basically, they are powerheads on steroids. They have a very broad pattern to their output, instead of just blasting a small powerful stream like most powerheads. You can also hook them up to a special controller that will control them for different wave and tidal patterns. They're pretty amazing - a 6000 stream moves 1800 gph and only uses 15w of power.
I didn't consider them at first because I was on a "no powerheads in the tank" kick. But, for a closed loop, you end up with lots of plumbing in the tank anyway, and it's a lot more work to setup. If the sight of them bothers you, they make decorative rocks to hide them. That's the route I took - once I aquascape the rest of my tank around the Tunze rocks, and coralline grows over them, you won't even know they're there.
Here's a picture of a stream inside a rock (it's the only thing in the tank right now, but you can see how well it could blend in once the aquascaping is done):