What visualization technique to best describe a recommendation dataset?
I think you're looking for a similarity matrix (see bottom of the page). If you don't have data on similarity between certain pairs, you can always leave them as grey or white. Also, this will only work for data sets small enough to actually make out what's going on. I'd say 25 rows / columns maximum.

In a similarity matrix, x, and y coordinates correspond to the two things you're comparing, while a colormap magnitude represents similarity 

EDIT:
One thing you could do to replace the colormap is the insert, say, circles of different sizes according to the similarity metric. Or you could insert the numbers themselves, again, varying the size of the number as the magnitude of that number varies. Size usually works best is business visualizations.