Answer:
   
   First and foremost, groups such as these often usurp Jewish
   terminology and practices. On the surface, the claim is that they do
   this because they are "completed Jews". For example:
     * They no longer call their places of worship churches, but use the
       words like synagogue, "Beit Knesset", "Kehillah".
     * They hide all the crosses and use Menorahs and Stars of David
       instead.
     * They often parade around with Torah scrolls while praying to
       Jesus.
     * Many wear "kipot" (Jewish head coverings) and prayer shawls.
     * They celebrate Jewish holidays like the Passover Seder, but
       pervert their meaning by claiming that theirs (Christian) is the
       real meaning of Jewish customs and traditions. By doing this, they
       negate, distort, or ignore their real origins and significance.
     * They call their leaders 'Rabbis'.
     * They refer to Jesus as Yeshua, to make him sound Jewish.
       
   Why do they do this? Recall that such groups consider both the Tanach
   and the Christian Gospel as the word of G-d. In the Christian Gospel,
   I Corinthians 9:20, it says: "To the Jews I became like a Jew, to win
   the Jews. To those under the law I became like one under the law
   (though I myself am not under the law), so as to win those under the
   law. To those not having the law I became like one not having the law
   (though I am not free from G-d's law but am under Christ's law), so as
   to win those not having the law. To the weak I have become weak, to
   win the weak. I have become all things to all men so that by all
   possible means I might save some." [New International Version]
   
   In other words, they're trying to sell Christianity in the guise of
   Judaism.
   
   Christians believe that through faith in Jesus a person is saved. The
   Christian has the perspective that this is such a singularly important
   thing, above all and any other thing in life--or, for that matter, in
   death--that any regard or concern for others (which is a fundamental
   part of his commitment to and love for G-d) that he persuade others to
   at the very least Hence, there is a strong emphasis to save people by
   bringing others to Christianity. For many of them, messianic
   synagogues are the answer. This allows, in their words, "Jews to
   become Christians while still remaining Jews and cherishing their
   Jewish ancestry. Swedish Americans cherish their Swedish ancestry.
   Chinese Americans treasure their Chinese ancestry. Hebrew Americans
   ought to cherish their Hebrew ancestry and remain strongly Jewish.".
   Missionaries proclaim, "I am a Jew. I go to synagogue. I invite you to
   my synagogue. It meets on Friday evening." They then make those to
   attend those "synagogues" feel right at home. But recall their mission
   statements: their goal is to bring the Jew to Christianity.
   
   Note that it is not just Jews that feel the practices of these groups
   are deceptive. The following are some quotes from a FAQ on such groups
   by Yad Lachim ([5]http://www.yadlachim.org/messianic/messianic.htm):
     * In 1977, the Board of Governors of the Long Island Council of
       Churches (New York) accused "Jews for Jesus" of "engaging in
       subterfuge and dishonesty," and of "mixing religious symbols in
       ways that distort their essential meaning." "Jews for Jesus" filed
       a suit in a State Supreme Court in Manhattan against a 600-member
       Council. The Rev. Jack Alford, the executive director of the
       Council, said the suit "proves the point we were making about
       their tactics." He added: "The mentality of 'Jews for Jesus' is
       the kind of mentality that has been spawn in some fascist and
       communist countries." Eventually, the lawsuit was rejected by
       court. (The New York Times, July 2, 1977)
     * In the summer of 1987 in Washington D.C. (USA), there was held an
       Interfaith Conference of Metropolitan Washington. Partaking in the
       conference were representatives of various Protestant churches,
       Roman-Catholics, together with Moslems and representatives of
       Jewish organizations. The Conference concluded with an official
       statement (published in "Interfaith Connector" Vol. 8, No. 2)
       which stated:
       
     "We condemn proselytizing efforts which delegitimize the faith
     tradition of the person whose conversion is being sought. Such
     tactics go beyond the bounds of appropriate and ethically based
     religious outreach.
     Examples of such practices are those common among groups that have
     adopted the label of Hebrew Christianity, Messianic Judaism, or
     Jews for Jesus. These groups specifically target Jews for
     conversion to their version of Christianity, making claim that in
     accepting Jesus as the savior/messiah, a Jews 'fulfills' his/her
     faith. Furthermore, by celebrating Jewish festivals, worshipping on
     the Jewish Shabbat, appropriating Jewish symbols, rituals and
     prayers in their churches, and, sometimes, even calling their
     leaders 'Rabbi', the seek to win over, often by deception, many
     Jews who are sincerely looking for a path back to their ancestral
     heritage.
     Deceptive proselytizing is practiced on the most vulnerable of
     populations - residents of hospitals and old aged homes, confused
     youth, college students away from home. These proselytizing
     techniques are tantamount to coerced conversions and should be
     condemned."