I would recommend a recent translation (Wilson's is reputedly very good - unlike translations in the public domain - but I don't know how his annotations and critical material is regarded, or if in more recent printings he brought it up-to-date with 21th century Japanese research on the book. I read it - excerpts in an anthology of works on the bushidô, in fact - in French, so I can't really help with that). The first scholarly edition (with an historical essay longer than the book itself) has come out in French only earlier this year, but I haven't got the chance to read it yet.

Historians of Japan (native and foreign) have a lot of misgivings about the Hagakure when if it's taken out of context, especially in their more recent works (you won't find those comments much in history books written before the 1990s. In the post-war years they simply discredited the book but for political reasons, nowadays it's challenged/analyzed more on scholarly grounds).

In the years after the war (and after the translation of Mishima's book that discusses it and made it known to westerners), the book has gained popularity in the West (especially in the US) where it was presented as a work that gives much insight in the thought and philosophy of the Japanese warriors. But scholars in Japan also consider this book as a main culprit for many modern misconceptions (abroad and among Japanese, not the least Mishima Yukio, and manga and movies etc.) about the samurai and the bushidô. Japanese nationalists in the 30s have used it massively for propaganda (so yes, it was influential to many soldiers and even more officers who fought in WWII, but they were deluded that it presented ancient and traditional ideas).

It's since been well established by modern historians that this book isn't representative of the widespread, mainstream Japanese ideas of its time (among the Samurai class itself, or the buddhist schools), and much of the "historical facts" and concepts discussed by the writer, seen today as a very reactionary fanatic at odds with people of his time, are fanciful (glorified facts rather than the truth, or truths interpreted to fit his political agenda), while philosophically it was considered highly unorthodox even in its time (and often unsound when it talks about philosophical concepts from earlier eras). It's presented today as one of the best documents(and the most extreme of its kind) to understand a fringe movement among the opposition to the modernization of the country, in reaction to neo-confucian reforms introduced by the Tokugawas, and its later influence on the Meiji and post Meiji nationalist movement.

The book is surrounded with "legends" (originating in the Meiji era, when the book resurfaced) and half-legends. There's this story that it was kept "secret" for centuries by the writer's clan, taught only to their own samurai. There's also this story (the samurai had the book written by a scribe after the lord of his clan forbade him ritual suicide, and before being a philosophical work, it was a political anti-Tokugawa manifest). that junshi (seppukku, following your liege's death) was "traditional" among samurai in the pre-Tokugawa era. It wasn't. It existed but was never a mainstream belief or practice (it comes from pre-historic and antique rituals, abandonned long (millenia!) before there was a warrior class). It's a "fad" that developped in the first century of the Tokugawa rule, one Edo nicked in the bud before it became popular by severely banning the practice.

Modern historians have established that the writer's Lord was actually so shocked by the book's views that he refused it be read (and it stayed that way for a century or so), while the shôghunate, to which the daimyô had sent the book, forbade its publication (seing it rightly as an attack on the neo-confucianist reforms it sought to introduce to civilize and pacify the warrior class, presenting a so-called "purer view" of the bushidô that actually never historically accurate. The buddhist schools (the Amidists, especially, but also the Zen) also objected immediately to the writer's unorthodox views on death.

In the Meiji era, when the Tokugawa era was seen very negatively, that real and half-imagined history of the Hagakure contributed massively to the book's popularity, and it was presented as the real bushidô, that the shôgunate had destroyed and perverted. In the post-war years, the book was massively discredited in Japan for having contributed to the ideas that lead to the war.

In the last two-three decades, the modern Japanese historians' views of the "Pax Tokugawa" have become a great deal more nuanced and far less negative (and most importantly, they largely broke away from the earlier tradition of highly politicized writing of History in Japan), and a lot of idealized/nationalis notions about the pre-Tokugawa era and the rise of the samurai class are torn to pieces by modern research and archeology.

The best works on Japanese History by western scholars (the classics, like Sansom's awesome three-volume History - that I would still warmly recommend for its narrative qualities) are (alas) considered quite outdated when it comes to historical interpretation and his surveys of Japanese historiography (but Samson tried to stick to narrative and was very cautious about interpretation, so his book on the early modern era isn't much tainted by the anti-Togukawa historiography still prevalent in the 60s when he wrote). There's an excellent (and also big) "New History of Japan" that came out last year and which makes abundant use of recent Japanese research (and it's probably the most interesting and most comprehensive Japanese history book I've read since Samson, and very good "companion" to his more detailled but less interpretative trilogy, I would say) but it's by a French historian and if it gets translated in English it won't be for a while.

What I've read of the Hagakure was really quite interesting (and I plan to read the whole thing in the new translation), but it's one of those books that need context.