1: I've seen various comments that this is a document prepared by someone who claims to be a former British intelligence officer. That's not right. If nothing else the CIA knows who served in the intelligence organization of a close ally. There's no reason to doubt that this former MI6 officer is just that and that he's someone who the US Intelligence Community has judged reliable in the past. That alone tells us pretty little. But I see no reason to doubt this specific piece of information.
2: The published reports say that the US Intelligence Community finds the former British intelligence officer credible and that they've spoken to or investigated some of his sources and also found them credible. I think what we should draw from that is that US intelligence personnel have looked at this and found that the sourcing wasn't obviously ridiculous, no red flags that would make them dismiss it out of hand. In other words, this is a very low threshold of credibility. This is nothing like the judgment multiple intelligence agencies have made about Russia being behind the hacking campaign. Just as clearly, no one in the US intelligence world has vouched for any of this being true.
3. What seems to be the case though - and this I take from a number of sources - is that various details in this document are being taken very seriously by the people who are paid to protect the country from foreign subversion, blackmail, cooptation, etc. That's worth knowing because there are certain claims in this document that strike me as highly exuberant and highly implausible. The overall tone of the document reads to me like a raw report in which solid information and highly questionable information is all piled in together. I say that not as anyone who has familiarity with intelligence work, simply as a journalist who has many times been shown opposition research, which is actually what this is.
4. The most salacious details in the document probably can't ever be confirmed or refuted. Mainly that doesn't matter though since they're really not relevant. But the document is full of details that should be fairly straightforward to check. Hotel stays in specific hotels on specific or near specific dates by public figures. Those are bits of information that should be ascertainable - if not by journalists than certainly by law enforcement and certainly by the people who control the vast information vacuum created by American spy agencies. That seems like the most concrete thing to do.
5. Similarly, there are specific claims about internal conflicts within the Russian government - people who wanted more aggressive or less aggressive attacks on the US election process. Those are the kinds of details a US spy agency might well know. If what is claimed in the document matches what US spies know from human or signals intelligence that might be why American intelligence agencies are taking this report seriously.
6. Some people are saying that we shouldn't be looking at these news reports through the prism of the credibility of the document but rather the intelligence community's retaliation or warning to Trump. I don't think we can rule that out. But as much as we may dislike Trump, intelligence agencies using the information they gather against an elected President is a very bad thing. We shouldn't lose sight of that. Of course, we should also not expect our spy apparatus to cover for presidential wrongdoing or leave the country vulnerable to foreign subversion. Regardless this is extreme high-wire behavior for the whole country, whatever the underlying facts. It's bad from every different direction that we're here.
7. A lot of the information in this document matches up pretty well with circumstantial evidence about Trump's business dealings with Russian oligarchs and organized crime, and the leverage they have over him.