Solr/Lucene 5 is a major release with lots of features and bug fixes. It is backward compatible with Solr4.x, but if you wish to upgrade from 3.x or older, you need to first upgrade to 4.x if you want to avoid reindexing all content.
The main news for Solr in 5.x is start scripts and installer script for Linux. You can now also throttle bandwidth of index replication. 5.1 brings an API to change common solrconfig.xml parameters during runtime. There is also a new JSON API for faceting and querying. See CHANGES.TXT for details.