It's certainly no more of a placebo effect than tweaking XP or Vista services, it has a definite impact on responsiveness for the desktop, it marginally reduces boot times and shutdown times, it reduces the number of threads actively running in the background and so that means less things running that could potentially cause slowdowns or conflicts, you can of course over tweak your system which ultimately will have a negative impact, since some of the more recent additions like Superfetch are helpful.
Like i said before though, do you think that somehow MS has discovered a way to use all these extra resources and run the extra services without it impacting the OS' overall performance? Win 7 has more overhead than any other OS that came before it, reducing that overhead can only be a good thing.
Anyone who doesn't think so or isn't sure should test it for themselves, i can't really recommend the application that i originally posted, since it seems to cause problems with some configurations, but it can still be handy just to make a backup of your default services in case you ever want to revert back, i would however recommend using Blackvipers site, go through your services one by one and check on his site what those services are for and if you need them running, then just disable those you don't need and others you can set to manual so they only run as and when they are needed, then see what you think, i know it's not placebo, i've too many people ask me to tweak their systems up and too many people telling me how it feels even faster than when they first bought it.