Today is the 100th anniversary of the completion of the Mishnah Berurah (link). At the end of the final volume, the author/editor -- R. Yisrael Meir (Poupko) Kagan -- writes that he finished it on the 19th of Marcheshvan 5667 (1906). The Mishnah Berurah is an important compendium and summary of explanations and rulings on the Orah Chaim section of Shulchan Arukh (link). The commentary has risen to the level of a standard and essential text. However, because it is so easy to overstate the work's importance, let me add some words to the contrary.
The Mishnah Berurah is not the final word on any matter, even though it is sometimes used as such by people incapable or uninterested in digging deeper. R. Hershel Schachter would often say that he never owned a Mishnah Berurah until his wife bought him a set after he was married. His point, I believe, is that it is not an essential work and one can study halakhah without referring to it. However, R. Schachter does quote the Mishnah Berurah on occasion.
Here is a case in point regarding hyperbole in which great praise can be misunderstood. R. Chaim Kanievsky, in his introduction to Shoneh Halakhos -- a summary of the laws in Mishnah Berurah, writes: "The book Mishnah Berurah has merited being accepted throughout Israel and is the final decisor in our generation regarding the laws of Orach Chaim..." Yet, later in the same paragraph, R. Kanievsky writes: "And in places where the Mishnah Berurah and the Chazon Ish disagree I listed both views." Well, which is it? If the Mishnah Berurah is the final decisor, why did R. Kanievsky quote his uncle's disagreements with the work? Clearly, he did not mean that the Mishnah Berurah is literally the final decisor. He only meant that it is a very important work.
R. Yehuda Henkin has a responsum on the subject of whether the Arukh Ha-Shulchan or Mishnah Berurah receives precedence in deciding halakhic issues (Bnei Banim 2:8). He writes:
In my opinion, my grandfather [R. Yosef Eliyahu Henkin] did not say that [we follow the Arukh Ha-Shulchan over the Mishnah Berurah] except where the Arukh Ha-Shulchan and the Mishnah Berurah disagree based on their own thoughts since the Arukh Ha-Shulchan was certainly sharper, was the master of the entire Shulchan Arukh and also saw the book Mishnah Berurah [yet still disagree with it]. See [Arukh Ha-Shulchan] 11:22, 12:4, 25:23, 22:4, 268:6 and other places where he refers to the Mishnah Berurah, and in 79:11 and 319:22 he disagreed with him, and there are many other instances. And in countless places he disagreed with him without mentioning his name. Two examples are 55:20, where he intended to rule against the Mishnah Berurah (there, no. 52), and 370:13, where he intended to rule against the Mishnah Berurah (there, no. 27), and this is clear to someone who delves into the matter. Therefore, it is a mitzvah to publicize that the Arukh Ha-Shulchan is not just a book unto itself but a reaction to the Mishnah Berurah. It is also a testimony to the value in which the author of the Arukh Ha-Shulchan held the Mishnah Berurah, even though it was a new book in his time (as one scholar pointed out to me). Also, my grandfather [R. Y.E. Henkin] was a great expert in the Mishnah Berurah.See also the words of Dr. Haym Soloveitchik regarding the Mishnah Berurah and the Arukh Ha-Shulchan: link
I find the Mishnah Berurah to be a great starting point but that's all it should be. And even then, someone unfamiliar with the laws will have a difficult time learning them for the first time from the Mishnah Berurah. I pity the person who tries to learn the laws of Shabbos from it.
[As an aside, those interested in acquiring volumes 1-3 of Bnei Banim should try calling Beigeleisen in Boro Park (718-436-1165). Volume 4 can be purchased online here.]