It's still considered an important, even seminal book, and it's one of the first books you would read as a historian specializing in the Holocaust. Obviously it's an old book, but it's still essential reading because of its importance within the historiography of the Holocaust, because it was really the first comprehensive study of the Holocaust in the historical literature. Hilberg's work founded what's known as the functionalist school of Holocaust historiography, which argues that the Final Solution developed gradually as a result of a succession of actions at the lower levels of the Nazi bureaucracy, rather than being directed from the top down based on a premeditated plan by Hitler (the latter is known as the intentionalist school). As a result, Hilberg was hugely influential on many of the next generation of prominent Holocaust historians, particularly those like Christopher Browning who were part of the functionalist camp. The functionalist-intentionalist debate isn't as salient as it was a few decades ago, as most Holocaust historians have come to a synthesis of the functionalist and intentionalist positions, recognizing that the Final Solution was the result of a gradual radicalization of Nazi Jewish policy, but that this radicalization was the result of Hitler's influence, even if he didn't have a master plan. The most extreme interpretations of functionalism and intentionalism don't have much currency anymore because we have greater historical knowledge that allows for more nuanced understanding of those questions, but it took the foundational works like Hilberg's to get those debates started in the first place.

Hilberg received some criticism from both scholars and the Jewish community as a whole for parts of his argument that were perceived as insensitive to Jewish suffering or outright offensive, particularly as regards his discussion about the Jewish Councils (Judenräte), whose actions he implied were tantamount to collaboration. He was also criticized for writing a largely bureaucratic history, rather than focusing on the suffering of the Jews or their experiences within the concentration camps, although it's worth noting that the source material he would have had for such a discussion would have been a lot more limited than it is now. Additionally, Hilberg was working primarily from German-language documents, so that also limited him to a more top-down approach that was centered on the Nazi regime rather than the Jewish victims (although it's worth noting that he later introduced the common "perpetrator-victim-bystander" model as well, and wasn't entirely focused on the perpetrators). It would have been basically impossible to write an overarching history that adequately covered every angle of the Holocuast, since there's no way a single person could speak every relevant language and do the necessary research in every individual archive (especially considering that much of the vital material that's gone into subsequent studies wasn't available to him because archives within the Eastern Bloc and Soviet Union weren't accessible at the time). Even in its existing form, Hilberg's work was a massive undertaking that's rarely been matched in scope or length since.

Inevitably, in the following decades, historians have critiqued various parts of Hilberg's work and found inaccuracies or oversights in the book, but it's important to remember that he was essentially starting a new field of historiography from scratch. There were only a couple of other histories of the Holocaust at that point, none of which had been widely read, so he was really covering completely new territory in historical research, and it was inevitable that he would overlook some things or make some errors. Of course, this is all part of the scientific process: you do your research, publish your conclusions, and your peers replicate your research and critique its shortcomings while adding their own insights, and then that process repeats itself on down the line. Theories are proposed, then met with counter-arguments, and then those positions find a synthesis, as was the case with the functionalist-intentionalist debate that stemmed from early studies including Hilberg's. The whole reason we have such a depth and breadth of historical knowledge about the Holocaust, from high-level studies of the leading figures in Nazi regime down to microhistories of specific communities, is because historians like Hilberg laid down the initial foundation that those later works built on.

So the tl;dr version is that Hilberg's work is dated in some respects, but that's inevitable for a work that was written so long ago, and it's still worth reading both for the information that's provided within the book and as a starting point for understanding the historiographic debates that have developed in Holocaust studies following its publication.