Indeed, it sounds very nice. Perhaps you can send us some pictures you produced with Mondrian :).
The most spectacular ones are:
http://infogroep.be/~tverwaes/visualization.gif
which shows different clusters and the parts of the hierarchies in a program represented by those clusters. Clusters have reasons to cluster elements together, and those boxes outside the box with the hierarchy represent those reasons. The bigger the reason-box, the bigger theamount of classes it represents. The darker the classes in the hierarchies are, the more reasons support this class in the cluster. If a class is green it means that a superclass as well as a subclass are present in the cluster, but not the class itself. I call these ghostclasses since they probably should be present in the cluster as well, but are not clustered by the algorithms. The white boxes in the top with yellow rays are classes not available in the cluster, but classes which are needed for the cluster to be usable (superclasses from which some classes in the cluster need to inherit). The gray ray means a cluster supports a class, while a blue ray means the whole subhierarchy is supported (a way to limit the amount of rays..)
http://infogroep.be/~tverwaes/visualization2.gif
something I am currently working on, which places clusters in the bigger picture. This specific picture clusters classes according to the namespace in which they were defined. The "best" layout available for something like this appeared the horizontalDominance, that's the reason for the funky layout.
Greetz.