Lucene is great. I wrote a proposal for cross-component search for Helm, blithely assuming that there was only one rational way to design a search engine; then panicked when I discovered that all the open source search engines were basically single-purpose scripts designed to crawl a web site and index the Web content; then looked at Lucene and rejoyced at its flexibility. It's designed as if implementing my ideas on the one rational way to design a search engine. How could I not love a piece of software that caters to my prejudices to such an extent?
It's also pretty fast at indexing, especially the way I plan to split up the indexes.
One search to search them all
One search to array them
One search to rank them all
And on the page display them