I love full text indexing. I use Lookout for Outlook and Yahoo Desktop Search on XP, and Spotlight (of course) on OS X.
The OS X experience is pretty much perfect -- especially compared to the XP story. Full text indexing on XP has all kinds of performance and usability issues, but the worst appear to be related to antiviral software
I can't find much about this on a google search, just hints that suggest I'm not the only one to notice this. The minor problem is that the antiviral software wastes cycles searching the text indices (files need to be exempted from av and backup). The big problem is that some antiviral software really trashes file i/o (I disable it when doing heavy duty database work), and full text indexing causes massive file i/o. So the combo on antiviral s/w and index building can bring a single CPU XP system to its knees.
Mercifully I run my OS X systems without that vile antiviral software, so I don't run into problems there.
I think this may be one of the reasons that Microsoft decided to take over the antiviral business. It really does have to be built into the OS in such a way that it interoperates with full text search -- in particular exempt some file i/o operations from the antiviral tax.