If you are using Sphinx on a rails project, I strong recommend Thinking Sphinx(http://freelancing-god.github.com/ts/en/). We used to use Ultra sphinx plugin, but thinking sphinx is more flexible.
We use Sphinx on http://esciencenews.com , and its performance is quite amazing. It powers search and the 'dictionary' (which is really tags) feature. Very easy to configure / deploy, too!
How does this compare to solr, compass and other "search frameworks"?
I'm currently using Compass (Java wrapper around lucene), but that was just to get something implemented quickly.