Most of the negative criticism of this book I've seen move along two lines:
1) Lenin was a dictator, Stalin, purges, 100000000 zillion people killed by communists alone in my village, whatever.
2) Lenin is a total dick that only insults his opponents, never giving arguments to support his viewpoint.
First criticism is total bullshit and we shall just ignore it. Regarding the second, it is true that, at first, one might have that opinion... for a brief time. Those who maintain it either haven't read the book, or they haven't understood a single word. LWCAID gives an accurate description of what "left-wing communism" is, while thoroughly studying its presence among the european countries (mostly UK, France, Germany and Italy), and explaining why he considers this trend childish, unsuccesful and harmful to the Communist movement. He explains the problem Communist struggle must faces in the material conditions of every country he speaks about, and he gives some suggestions regarding how the parties shall carry their activity. He leaves no "insult" or calification without an argument and explanation behind. He acknowledges mistakes he and other Bolshevik comrades had made in the past, and he even comes to the point of publishing a letter from the Communist Party of Holland that refutes some criticism Lenin made to them in the initial draft of this book.
It is more than obvious than the text, despite being a pamphlet, is far from being an irreflexive attack on other Communist, and more of a warning against some harmful tactics and discourses. Lenin is pretty autocritical and has no problem admitting mistakes and constantly repeating why he thinks others are wrong, instead of just pointing out that they are so. It is, no doubt, a necessary reading for every marxist, and probably more so now in 2017, to analyze present conditions in the light of this text.