Well, due to some bugs/quirks in Spotlight, they haven't helped me nearly as much as I expected. But that's another issue.
The single feature I've used most is the ability to search by image width/height. Unfortunately you can't search by aspect ratio, but even so, it's a big help. I've been collecting desktop backgrounds since I used an 800x600 monitor. I've since moves to 1024x768 and now 1280x1024. Smart Folders/Spotlight really help me sort and clean out some older stuff I don't want anymore.
I also use it to (sort of) mimic my iTunes smart playlists in the Finder. But due to the bugs/quirks I mentioned before, it's not perfect. Still, it's handy.
That's really all I use it for right now, although I intend to set up some Smart Folders that make use of the new "where from" tag (which can be a folder if you moved it from somewhere else, or an URL if you downloaded it).