I am very impressed with Lucene's performance. Here is an interesting read though it doesn't talk about scalability in depth - How Digg.com uses the LAMP stack to scale upward
Digg's current architecture includes about 20 database servers, 30 Web servers, and a few search servers running Lucene; the balance operate as backup servers. All but one of the database servers run some version of MySQL 5. The transaction-heavy servers as well as the backup units use the InnoDB database engine, while the OLAP ones use MyISAM.
Joel mentioned that Lucene.NET a straight port of Lucene used in Lookout an OutLook search Plugin was blazingly fast while Microsoft's built-in search sucked.