Gridcoin is absolutely legitimate. There is a very passionate developer community working on Gridcoin driven by their desire to further scientific advances. Gridcoin advances science by partnering with BOINC, a program that distributes scientific work to anybody that has a desire to help process it. As you noted PiGrid has also recently gotten involved by creating a plug-and-play hardware solution with Gridcoin and BOINC preloaded and ready to go for anybody. Gridcoin is only two years old but people are really taking note of it, the word is getting out, and now is a great time to participate while it's still in the early stages. Also the growing "Citizen Science" movement has helped people to become familiar with Gridcoin. You already noted the links in your post, so get involved!