It has received considerable critical acclaim. I think this is true because, unlike many war novels, and especially those about Vietnam, the book does not simply explore one experience or perspective. And Vietnam, more than most American conflicts, I believe was very difficult to characterize in a novel format. This is why the collection of short stories works so well for me as a reader and a historian.
Yes, this book was very popular from its release dates, and critics rushed to applaud its depth of insight and knowledge and the kind of comments it makes about war and the experience first-hand of those fighting in it. #2 is right in highlighting a continuing debate concerning the precise nature of this collection of short stories or novel, as to how to define it, but apart from this, it has been immensely popular.
The Things They Carried was critically acclaimed upon its release in 1990. The only debate about the book concerned whether it was actually a novel or a collection of short stories. It has since been described as a "unified narrative" and a collection of "interrelative stories." One reviewer immediately labeled the book "a masterwork."
As soon as it was published, critics were calling it one of the most emotionally impacting novels of war. It's hard to argue against them, though there will always be critics that complain that maybe the material did not happen as exactly as written, it is fiction after all.