Does what it says, and flawlessly.
Always used SmartSaver on Snow Leopard. Went over to Mavericks and wanted something similar (SmartSaver is not compatible).
Two cons though:
1. Slider moves the opposite way from what you (I) expect.
2. Only available at the Mac App Store. So it immediately installs. Hate that! And no DMG or ZIP to backup.
Screen snapshots taken with the screen dimmed by Brightness Slider generates a correspondingly dimmed image -- unlike Shades which captures a fully lit image, though that app occasionally fails to launch when booting.
I surmise that most of these screen dimmers place a translucent shaded window as a top-most layer and take the snapshot above that layer rather than below it which requires one to remember to fully brighten the screen before capturing a portion of it!
This works just fine on my NEC monitor - much easier to use than the built-in onscreen monitor controls. Though I sometimes take a pass on software from the Mac App Store because there is no trial version available, because this app is free, there is no risk involved in getting it from the MAS.