This week the ß version of Apache Lucene/Solr was released, see release announcement. This means that things are getting stable and approaching the FINAL release – after a looong time in the makings.
The most important changes is perhaps better maturity for SolrCloud along with a lot of bug fixes. If you need the ease of scalability that SolrCloud brings, this beta is a safe version to get started with – even in production. We (the developers) promise that we’ll not break backwards compat unless absolutely necessary due to a bug or something.