Windows 7: game booster

^LOL yup useless. Besides you can always disable the services plus more yourself.

But some people might disable the wrong processes thus crashing there system ect. So gamebooster "can" affect your system, but it really depends on what kind of system you have.

It helps to an extent but only if your low on ram or your cpu bogs up with all the services open and all gamebooster does is close some services that windows opens that aren't necessarily needed when playing a game and leaves more room for your cpu and ram to handle. That's basically all it does.
It all depends on how much stuff you have running in the background. If you're running a pretty much stock setup you won't notice a difference but if you have a lot of background processes, especially ones that tap heavily in to windows resources, you are going to notice the difference.
Did a test with fraps on Borderlands and it gave me 5-7 fps boost, I use it all the time when I am gaming on my laptop.