Last summer Anthony Arnone and I did a study on the performance of three open source search engines.
Here's the Report in PDF.
Solr was the clear winner. Sphinx was in a close second with blindingly fast indexing times.
At this point the report's results are somewhat dated as both Sphinx and Solr are readying new releases. So your mileage may vary, and I'm sure Peter Zaitsev and the Sphinx team could show us how to improve the performance of their engine.
Updates: The Sphinx team contacted me and suggested some ways to improve Sphinx performance. New results will be published some time soon. They will likely also publish a test using Wikipedia as the document repository.
More Updates: I have started a new Solr project and may test Sphinx again.