It's speedy for the first few months, but once I build up a few dozen gigabytes of documents it starts to slow to an absolute crawl. Useful when searching for documents, absolutely, and faster than alternatives. But worthless for applications, which I open far more often than the average document (from a launcher, that is).

But that might be because my .Spotlight folder is > 1 gigabyte. And that's smaller than it has been in the past - my previous hard drive had it larger than 2 gb if I remember correctly, because I had tweaked it to index my source code. On my wife's computer it's only about 400 meg, and it finds applications in about a second (still much slower than Alfred or Quicksilver).