I installed Google Desktop for Linux last week, and it continues to impress. As an example, today I downloaded the ISO Schematron specification. I immediately unzipped it and opened the PDF. Before starting to read the PDF, I typed "Ctrl-Ctrl" to bring up the Google Desktop search box, and typed in some of the keywords from the opening titles of the specification. The PDF was already in the search results, which I guess means that Google Desktop is monitoring file/directory updates rather than continually performing searches of the filesystem.
That's good - it keeps the CPU load down, while keeping the search indices (indexes) up to date. I have no idea whether this is the approach taken by other search tools (like Beagle) or not.