You can see from the drop box what prefs are available. You can choose to have the FontSight menu appear under the Format menu instead of up on the menubar taking up space, which is a nice touch, and you can set it to group your fonts in families or in one long list. The families feature will make users of Adobe's Font Reunion very happy.
Also, there's a cool Preview Fonts button that brings up a floating Preview window allowing you to see samples of all the fonts at a specific size:
While all these features are nice, I was surprised to not see a Font Size menu included. This means (for Keynote users at least) you still need to have the Fonts Panel open, although you can compress it down pretty small and use the size slider to set your font sizes.
Since I was curious as to WHAT FontSight did to my system I did a simple find for anything with the name FontSight in it and discovered that FontSight actually makes images of each font and saves them in a cache folder inside the Application Support folder. Pretty cool trick.
While this is a great little utility, we'll wait for Stone Design to figure out how to add a Font Size menu to it before we shell out $19. Overall though, it's pretty handy, especially since it works in all Cocoa apps with little or no tinkering needed.