The severity and importance of the Mitzvah:
The reward for the Mitzvah of Tefillin is very great. The following statements are recorded by Admur regarding the greatness and importance of this Mitzvah:
A. Poshe Yisrael Begufan:
One who does not put on Tefillin [for even one day] is considered within the category of “Posheiy Yisrael Begufan”. [According to Kabalah, such a person is called a Karkafta that does not merit to be resurrected in the period of Techias Hameisim.] Even if he refrains from wearing them due to fear of not keeping a clean body, he has transgressed, as certainly he is easily able to guard his body during the time of Shema and Shemoneh Esrei. [Nevertheless, in such a case, some Poskim rule the person is not called a Posheia if he does not wear the Tefillin due to reasons of cleanliness.]
B. The Mitzvah of Tefillin is the deciding factor on judgment day:
The Sages taught that on the Day of Judgment, if one is [judged] meritorious in the Mitzvah of Tefillin then the side of merit dominates. However if one was negligent in the Mitzvah then the side of liability dominates.
C. Is the greatest positive command:
The Mitzvah of Tefillin is the greatest positive Mitzvah in the entire Torah, as the entire Torah is connected to Tefillin. For this reason every person needs to be careful in the Mitzvah of Tefillin.
D. Merits one long life:
All those which are careful in the Mitzvah of Tefillin will have lengthy days. [This is because the Tefillin corresponds to the heart and mind of which one’s life is dependent on.]
Sparks of Chassidus:
Chazal state that the Mitzvah of Tefillin is connected to the entire Torah. The purpose of all Torah and Mitzvos is to draw down G-d’s unity into the worlds. Although this is accomplished by all Mitzvos, nevertheless by the Mitzvah of Tefillin it is more revealed, as the Parshiyos inside the Tefillin discuss G-d’s unity. Hence it is the head of all the Mitzvos.
Surrounded by Mitzvos:
Rebbe Eliezer Ben Yaakov stated: Whoever has Tefillin on his head and Tefillin on his arm and Tzitzis on his garment, and a Mezuzah on his door is guaranteed not to sin, as the verse states “Vehachut Hameshulash Lo Bemiheira Yinatek” as well as the verse states “The angel of Hashem rests around those who fear Him, and save him from sin”.
 Admur ibid; Michaber 37/1
 M”B 37/2; P”M 37 A”A 1 in name of Nachals Tzevi 1
Other opinions: Some Poskim rule that one is only considered a Poshei Yisrael if he did not put on Tefillin his entire life. [Rambam Teshuvah 3/5; Shaareiy Teshuvah 3/11]
 Rav Chaim Vital in Shaar Hakedusha 1/7, brought in Kaf Hachaim 37/6
 M”A 37/1; See Machatzis Hashekel ibid; Beir Heiytiv 37/1
Other opinions: Some Poskim rule that even in such a case the person is included within Poshei Yisrael. [Beis Yosef in opinion of Tur; See Kaf Hachaim 37/5]
 This is learned from the verse “Lemaan Tehiyeh Toras Hashem Beficha” [ibid]