One of the hardest books to read and see that for a pittance the world could eradicate several types of horrific diseases or drastically improve large numbers of lives, through simple targeted investments, and yet it never happens.
Here's a more personal example. After reading the book, a friend who made a mint in the oil industry bought four mobile drilling rigs, fitted them for digging water wells, paid to ship them to a state in I can't remember which country, but it was a poor one in Africa, where he hired a crew to run the operation and dig wells for free. He said it was a nightmare of red tape to operate, but wells got dug and people got fresh water that used to get dirty water.
However, having read a bit about their approach, it isn't clear to me that the methods used in that process are particularly effective at assessing longer terms problems such as climate change (and this seems to be a common critique).
There are some interesting analogues between the limitations of the CCC approach and those of longtermism, in that both focus on timescales (respectively short and very long) that diminish the importance of climate change, which is most significant as a medium-term risk. Having said that, at least the CCC work has some methodological rigour on its own terms, and actual utility - as I noted in another comment the longtermism "methodology" seems pretty hopelessly naive, and uninformative.
Which does research into the most cost-effective ways to fight global poverty. Absolutely marvellous work.