I don't think our major intelligence agencies would rely on information from someone who they don't find credible. If the source was a bit more shady, it would probably still be under investigation, with the intelligence agencies trying to verify the information or find their own information on the matter.
Yes, the author of the intelligence reports about Russian hacking into the voting process is credible. The person who authored these reports is a former intelligence agent with the British intelligence service. He has continued his work as an agent in private practice, and has been determined to be a credible source by U.S. intelligence.
I think the author is credible, otherwise why would he type out 35 pages? I also think he meant to verify his claims further but maybe he didn't have time. Now that the memos have been made public, it's up to the intelligence community to verify or debunk them. The memos shouldn't be forgotten or swept under the rug.
The author of the intelligence reports about Russian hacking into the voting process is not credible. Really, there is no way to completely account for the veracity of this author, or people like him. The Spy vs. Spy game is riddled with inaccuracies. No way to actually tell what is the truth or not.