My good friend Bertrand has published an interesting hands-on article on Solr, a RESTish front-end to Lucene.
We've been using Lucene quite heavily lately on my current project (even if not with Solr). It really kicks ass: fast, easy to use, robust, scalable, etc. And by using its low-level "expert" APIs in creative way, you can build in no time some really powerful things that don't have much in common with what people classically have in mind with "search engine", and that would be quite hard to achieve with a SQL database.
Lucene rocks!