Title. But I’ve been interested in the Stalin-era of the USSR for some time, and out of the works I’ve been looking at to start with Kotkin’s work on Stalin really stood out to me as impressive and all-encompassing. I’ve watched nearly all of his lectures and am about to buy his books, but I wanted to make sure this would be a good starting point for my real deep diving into Soviet history. So, is this a good start or have those angry YouTube comments calling him a reactionary historian true?
Kotkin is a perfectly respectable historian. I haven't seen the youtube comments you're talking about, so I can't comment on them specifically, but he's definitely considered an academic historian of good repute.
All that being said, I would not recommend his biography of Stalin for a newcomer to Soviet history. It's really detailed. I realize that sounds good, but I am almost puzzled this book has been marketed to a general audience when to me it is almost a total specialist's book. Everything is zoomed so close in and there's such an assumption that the reader already knows the historical framework and debates. It's like looking at an elephant by watching a film that traces the elephant's entire body slowly through a magnifying lens. Sure, someone who really knows elephants will understand what parts they're looking at. But a newcomer will just see lots of gray skin (I assume—I'm not a biologist). Perhaps having watched his lectures first will give you a helpful framework; if this is your passion, go for it.
But I would recommend some other books on Soviet history over the Kotkin bio. Sheila Fitzpatrick is an amazing historian with a real flair for literary style. Her books Stalin's Peasants and Everyday Stalinism are gems about the period, and the latter is particularly readable. I also would prefer Kotkin's Magnetic Mountain, which is about the construction of a new city of Magnitogorsk as part of Stalin's industrialization plan. That one has some of the same detail issues as the biography but it's a more focused time and place, so it gets away with it I think. Katherine Smith's Moscow 1956 is a fascinating and very readable portrait of a pivotal year after Stalin. There are so many others as well!
If you're ever in a bind and don't know what kinds of histories and historians to trust, I recommend looking at book reviews in academic journals. You can find them through JSTOR (which will at least give you the first page of the review). The presence of such reviews indicates that historians take the book seriously and you'll get a sense of whether or not they think the book is at the least reliable.
Whatever you choose, good luck!