Kirkus was one of the major pre-publication review sources in the publishing trade for many years, along with Publishers Weekly, Library Journal and Booklist. Since it had a tendency to be snarky, a good review always made people very happy, a starred review exceptionally so, and a bad review could be ignored because after all weren't they all that way.
Even the Wikipedia page for Kirkus is terribly shallow, however, because nobody outside of the trade really cares. My recollection is that it had been family owned for many, many years. It was sold mostly to libraries and to publishers with very little circulation beyond. In a day when there were very few sources of broad coverage of books, it was a very important subscription for libraries to have. But then the internet comes along, and budgets for very expensive magazine subscriptions (for as little as Kirkus might pay for a review, they had to pay something and they reviewed many many books) tightened and the subscribers dwindle and it really is expensive. The family may have sold it several years ago to some other entrepeneur and I can't remember how it ended up in the hands of Nielsen. In recent years it tried to experiment with doing things like offering paid-for reviews but allegedly real reviews but still separate from the regular reviews for people who were self-publishing. I guess if I wanted to look for citations for some of these things I could update the Wiki page.
But bottom line is that it became more and more irrelevant, and while never a staunch supporter of sf/fantasy it became even less so in recent days. So as Andy Wheeler says, it's hard to get too upset that it's gone.
But here's why it's worth commenting upon. While not known to the general public it did have some resonance still in the trade and in libraries. There aren't many places like that, especially as newspaper reviews have become fewer and fewer. And you can certainly say of PW what one will say of Kirkus. All of these are under deep attack in the internet era, and none of them are crucial to anyone. But if we are left with none of them at all, if all we have are the dozens of internet review sites which are important to very few but without meaning to the world at large, it's going to become harder and harder to get a book known and heard about, to build buzz and get things to rise above the crowd. Even in its last days, telling people in-house that you have a good review from Kirkus does more than telling them it has a good review from [insert your favorite internet review site here].
And that's not a path I'm thrilled to be travelling.