The Apache Sling team recently announced the first official release of Sling. Now you can download some nicely packaged Sling bundles to play around with.
I have been experimenting with the Sling/CRX bundle that came with Day Software's JCR Cup 2008 competition (entries due midnight September, 30) and was really impressed by what I saw.
Despite the fact that Sling is still an incubation project, it is fairly mature and robust. Day's upcoming release of CommuniquÃ© (version 5) uses Sling extensively. I envision Sling being used in a presentation tier where pages are statically rendered (baked) from content in the JCR and Sling is used to power dynamic AJAX overlays using content from replicated JCR workspaces.