I took a look at the 1995 Harvard study. It shows that if you sit in front of a piano keyboard for two hours a day while a researcher carefully monitors whether you are mentally practicing a skill --without physically moving your fingers -- you show SOME degree of learning -- but not as much as a group of people who actually did the physical practicing. This is very, very different than the implicit claim that merely imagining something makes that something more likely to happen. In fact, I'd much rather practice the piano for two hours daily than sit still for two hours mentally practicing the piano (again, for two hours daily).
It is dangerous to suggest to people that visualization can change one's life without clarifying exactly how much visualization one has to do -- and what kind. Visualizing a piano sequence for ten laborious hours can make me a little better at the piano than doing nothing. But actually playing for 10 hours is better for me -- and it's more fun.
It is disingenuous to pass this study off as advice about things like visualizing a better future. It IS good to surround yourself with happy people if you can. Studies show that very clearly. But it is not at all clear that simply visualizing a better future will make my real future any better. In fact, some work suggesting that this was the case (the "power pose" research, for example) was shown to be deeply flawed.