I use it, and it actually works without any real risks that would prompt someone not to.
What it basically does is that it stops and processes or services that aren't required for playing games (configurable, in case there's something you want to keep running), cleans up as much RAM as it can, clears any temporary data, tweaks the computer's power settings to squeeze out as much performance as possible and dynamically alters process priorities to focus power on the game while pushing pretty much everything else aside. I get an average of 50% better performance in games when using it, which is a pretty noticeable difference.