This week I ran a test of implementing docValues for facet and sort fields in Solr. docValues are a third way of storing data in Solr and are meant to be a solution for poor facet performance on large sets of results. docValues are "column-oriented", which means that they use documents as keys rather than terms and they are built at index time rather than traditional facet creation and counting which is term-oriented and done at query. You can read the documentation at lucene.apache.org (link above) for a better explanation of the mechanics than I could provide.
In short, docValues appears to be a performance gain while using less system resources, so a bit of a win-win I think. There are still some bottlenecks left to solve in our query performance, and there is a solid chance that the test I setup had some serious flaws, so please let me know if you have any feedback.
Report (Word doc)