A number of prestigious people from AI and other fields have signed the open letter, including Stuart Russell, Peter Norvig, Eric Horvitz, Ilya Sutskever, several DeepMind folks, Murray Shanahan, Erik Brynjolfsson, Margaret Boden, Martin Rees, Nick Bostrom, Elon Musk, Stephen Hawking, and others. I'd edit that into the OP.
Also, it's worth noting that the research priorities document cites a number of MIRI's papers.
After the first few pages of signatories, I recognize very few of the names, so my guess is that LW signers will just get drowned in the much larger population of people who support the basic content of the research priorities document, which means there's not much downside to lots of LWers signing the open letter.
I'm impressed they managed to get the Big Three of the Deep Learning movement (Geoffrey Hinton, Yann LeCun, and Yoshua Bengio). I remember at the 27th Canadian Conference on Artificial Intelligence 2014, I asked Professor Bengio what he thought of the ethics of machine learning, and he asked if I was a reporter. XD
This has appeared in the popular science page "I Fucking Love Science", followed by almost 20 million people on facebook. I think this is extremely good news. Despite the picture and its caption, the article seems to take the matter seriously.
The Future of Life Institute has published their document Research priorities for robust and beneficial artificial intelligence and written an open letter for people to sign indicating their support.