I just hooked up the Tunze Osmolator Universal 3155 and give it 2 thumbs up.

Pros: Easy to set up, to the point that it's one of the few retrofit type gizmos I've used that have worked on the first setup. The unit is quite instinctive on how it works and the control box is a complete no brainer.

Cons: More of those instructions that are multilanguage and multiproduct.
The definite con though, is that the holders for the water sensors, are geared more for sumps/aquariums where the walls are FLAT, ie. none of the plastic bracing rims that go around aquarium tanks. Otherwise this problem, can be worked around with a bit of imagination.