I have four of their albums (Mental Modulator, Spaceglider, Wellenbad and Distanz), and I like them all, especially Mental Modulator, which is a real classic. Distanz isn't quite as good as the others though.
They can be quite uneven, but at their best they make excellent minimalistic trance, techno or whatever you choose to call it. The best tune I've heard from them is probably "Stella", from Mental Modulator. Simply awesome!
They've also released some stuff under the alias Dr. DNA which is equally brilliant, check it out if you haven't already.