LR 4.2 includes a variety of minor bug fixes, plus support for 10 additional camera models. See the Lightroom Journal for the full list of changes. ACR 7.2 adds support for the same cameras, plus support for almost 30 lens, including some of the popular third-party lens for Canon and Nikon DSLRs. The complete bug list for ACR 7.2 is available at the Lightroom Journal.
Both release candidates have been privately beta tested and are quite stable. Adobe traditionally holds off on dubbing them final versions until the wider community of public testers can put them through their paces.