There’s an interesting conversation about hash clusters growing on the OTN database forum at the moment; I’ve made a couple of contributions to it, and think it’s worth reading.
For some other comments I’ve made about hash clusters in the past:
Hash Clusters – 1: also references a link on OTN where I’ve made a couple of comments
Sorted Hash Clusters: An introduction to a relatively new, interesting, but (at the time) buggy feature
Sorted Hash Clusters – 2: A closer look at what goes on under the covers.
Manual Optimisation – 3: Where I link a dirty trick for optimising a query with the optimizer’s “dirty tricks” with sorted hash clusters.