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QS is no replacement for Spotlight. QS's index does not look into the contents of files. You have to know the names of files to search them with QS. Also, you can't do dictionary or calculator stuff with QS.

QS itself, however, is an amazing program. I call it Godsend. It is lightning fast, so it's the ideal program launcher or file launcher when you know the file name (which should be most of the time). Personally, I have only my ~/ (home) folder and my /Applications folder indexed with QS.

It's also nice how you can search your files and do other "actions" on them with QS. Pretty much, what QS does is bring the icon of the file you search to the front of the screen. Other than that, it has some nifty features like the ability to pause/play/next control iTunes and QT as well as execute Terminal commands.

I highly recommend it. Just turn off the special effects; they tend to crash the program.
 
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