Many people have heard the term "Messianic Judaism" and are unaware of exactly what it is. Further adding to the confusion, someone who has little knowledge of mainstream Judaism may interact with individuals or writings in the Messianic faith and have little means of distinguishing it from what most Jews consider the central tenets of Judaism. It is important to understand that Messianic principles are not traditional Jewish principles. This is not to say they are bad, only that they differ much more significantly than the name suggests.
In short, Messianic Judaism is the application of Jewish rituals and aesthetics to a core belief in the divinity of Jesus and his role as the savior of humanity. From any informed perspective, this is simply another form of Christianity. No matter what, the concept of a messiah who has already lived runs counter to the very highest principles in Judaism. Christianity by any other name is still Christianity. Messianic Judaism is no more Judaism than a tigershark is a tiger.
In order to understand what Messianic Judaism really is, one must understand its history. Modern day Messianism grew out of the Hebrew-Christian movement of 19th century England. It is important to remember that Jews were, without question, considered a distinct ethnic group until roughly the middle of the 20th century, and even then the shift toward pure religion or culture happened largely inside the US. For practicing Christians who were considered ethnically Jewish, there was something of an identity crisis. The solution, however short-term, was to integrate the face of Jewish culture into solidly Christian philosophies. In that sense, Hebrew-Christianity was a lot like Greek Orthodox Christianity. The language and decor was of a particular ethnic flavor, but the religion was mostly the same.
When Hebrew-Christian concepts made it to America, the Messianic Judaism movement grew from it. So, while Messianic communities integrate Hebrew into their churches and services, keep the kosher laws and observe a Friday night-Saturday morning sabbath, their central beliefs are undoubtedly Christian.
Many practicing Jews, myself included, are vocal in their objection to Messianic Judaism, or at least the term. First and foremost, because the term "Judaism" is present, the adherent philosophies of Messianism are sure to mislead others into thinking that traditional Judaism accepts the idea of Jesus as savior. More disturbing is the missionary intent of this sect. The name "Messianic Judaism" is very likely aimed at mainstream Jews as a means to a missionary end. At best this is a misguided attempt to introduce Christian philosophy into the lives of practicing Jews and at worst it is a conscious attempt to convert by subterfuge.
All current sects of mainstream Judaism, from the ultra-orthodox Chasidim to the most progressive side of the Reform and Reconstructionist movements, recognize Messianic Judaism as being Christianity in all but name. There are a few voices that run contrary to this stance, but they are an extreme minority. Without exception, the Jewish faith simply cannot include the concept of a Christ-like messiah.