Apache Lucene is one of the most interesting search api's in open source marketplace. it provides a powerful and pluggable interface to provide full test feature set to many java based application.
but Lucene is "only" an api not an application.
Apache Solr is on the other hand is an enterprise search server based on Apache Lucene. the feature set behind that solution is worth to look at, means Solr introducing features some commercial systems still missing. for example using standard enterprise search provided by MS within their Sharepoint product you wonder how less features are available there.....(maybe adding FAST into next generation of Sharepoint change that but thats another story),
since few month professional services for the mentioned open source products can be used from Lucid Imagination. providing support and integration work for open source products makes the usage for the products more useful for the enterprise.
if you have to provide advance "full text" feature double check the open source community before you invest in commercial variations....