The original Tikkun was established by the ARI HaKadosh (Rabbi Yitzchak Luria; "ARI" is an acronym for "Ashkenazi Reb Yitzchak"), and is basically an abridged version of the entire Torah. It includes excerpts from all 24 books of the TaNaCh (The Pentateuch, Prophets, and Writings), taking several verses from each individual section and entire pieces of significant events in the Pentateuch (e.g. Creation, the Exodus, the Revelation at Mount Sinai, etc.). The Tikkun continues into the Oral Torah, with excerpts from each tractate in the Mishna (here's where things get a little confusing: I've heard that the Tikkun of the ARI doesn't contain any Mishna, only the Tikkun of the SheLaH Hakadosh has anything from the Mishna - I don't know, but I use the Tikkun of the SheLaH myself, and there is indeed Mishna there...), and then focuses on the esoteric sources, quoting Sefer Yetzirah (an ancient book that is attributed to the patriarch Abraham and contains secrets about Creation utilizing the power of the Aleph-Beis) and the Zohar. The Tikkun ends with the enumeration of the 613 commandments as compiled by the RaMBaM and the recitation of the entire Shir HaShirim (Song of Songs).
Reb Shimshon Pincus, OBM, gave a fascinating shiur in which he discussed the Tikkun at length, describing it and relating a famous story that occured between the Vilna Gaon and the Maggid of Dubno:
The Maggid once visited the Gaon for Shavuos, and they went to the shul to learn. The Gaon pulled out a Tikkun Leil Shavuos and began reciting it, while the Maggid pulled a gemara (a volume of the Talmud) from the shelf and started studying. When the Gaon asked him why he wasn't saying the Tikkun, the Maggid replied with a parable, likening the Gaon to a merchant who has a massive storehouse of goods, replete with every imaginable product, and the Tikkun that he was reciting as the small display of samples that he uses to advertise to the world. For such a merchant, the sample box is fine, because he has a storehouse from which he can summon any product - but for a merchant who has nothing in his storehouse (i.e. the Maggid, in characteristic humility), what good is a sample box? Better to just advertise what he does have.Rav Eliyahu Kitov suggest the possibility that just like the general custom of learning the whole night of Shavuos is meant to serve as a rectification to the B'nei Yisrael's sleeping in on the morning of the Revelation, so too, the Tikkun serves the same purpose. It is our way of showing that we are eager to receive the wonderful gift that God is about to bestow upon us, by previewing everything that it contains...
The Kav HaYashar (the "Just Measure" or "Straight Line", by Rav Tzvi Hirsch Kaidanover) has some powerful words to say about the Tikkun (end of Chapter 93):
And on the fiftieth day Israel became cleansed of their filth and entered the last of the fifty gates of understanding, all of which are pure. The fiftieth day was holy and pure and on that very day Israel was granted freedom from the Angel of Death and the enslavement of the nations.The author alludes to some Kabbalistic forces at work during the recitation of the Tikkun that are beyond my capacity for explanation. However, we do see that there is enormous significance to the saying of the Tikkun, and that it is more than just a "nice thing to do".
Therefore, it is fitting for everyone who fears HaShem and trembles at His word to study all night long on the Yom Tov (holiday) of Shavuos, reciting the Tikkun composed by the disciples of the ARI. And to toil in the Torah, the Prophets, the Writings, the Mishna, and selected passages from the Zohar.
Some recite the Tikkun on both nights of Shavuos, and fortunate is the one who does so. For through the arousal from below one arouses a unification on high, resulting in a tremendous outpouring of holiness. In this merit may the Holy One Blessed is He pour down upon us a new light and may we all merit its illumination, Amen.
I'm not necessarily endorsing it (it certainly doesn't need my endorsement, either), but I can explain why I chose to begin saying it a few years ago, and what it accomplishes for me. It was first introduced to me many years ago by my good friend, Reb Y. while we were in yeshiva for Shavuos. Back then, I would learn, but it was a struggle to have a real good night, filled with quality learning - there were the inevitable coffee breaks, which led into brief bull sessions in the coffee room and eventually stepping out for a cigarette (which turned out to be a more complicated halachic matter than I was led to believe. But I digress..). I wanted more, I wanted to have a night that was completely immersed in the Torah, without the myriad distractions that tug at you in the wee hours of the morning.
I found that in the Tikkun. As a matter of fact, once I start, I can't take a break or pause for more than a few seconds if a want to actually finish it, because of the sheer volume of material that I am trying to cover. Some might sniff at the seeming "ease" of the recitation, especially when compared to in depth pilpul (talmudic dialectic and analysis), but this is what works for me.
I've never tried it on both nights; after seeing the Kav HaYashar, though, I'm tempted to try this year.
The Tikkun Leil Shavuos can be found in any Jewish bookstore.