First and foremost, someone who stays with Daf Yomi for an entire cycle has the opportunity to see the entire length and breadth of the Talmud, the central source for Jewish law and Rabbinic wisdom. While such a breakneck pace is not ideal for retention, it is impossible for some of what has been learned to not stick, and for those who teach Daf Yomi classes, the rate of retention is undoubtedly much higher.
Second, Daf Yomi is an amazing exercise in self-discipline. It is one thing to have a chavruta (study partner) that you meet with a few times a week, assuming that both of your schedules can be made to work. It is something else entirely to go over 2,700 days in a row (that's more than Cal Ripken Jr., and he got off days and winters) and never miss that hour of Talmud study. This includes working around Yom Kippur and Tisha B'Av and business trips and vacations and weddings and the million and a half other things which make up our daily lives. To do it for a month or a year is already quite an accomplishment. To do it for a cycle is indeed praiseworthy.
There are many more reasons to recommend Daf Yomi and to praise those who take part in it. However, I want to pose a challenge to those who just finished and are thinking of getting started again (or perhaps already have). The challenge is to consider whether you can do better. Again, let me be clear that I am in no way denigrating Daf Yomi and all that a person accomplishes by doing it. However, for someone who has already completed one or more cycles, what is the reason to start over? Is it simply a reflex after so many years? It is a sense that everyone is doing it? Daf Yomi shiurim often take on the air of an exclusive club, with those who are involved forming a camaraderie which goes beyond their Torah study. If you are starting over, is it for the social benefits as much as for the learning?
Perhaps it is for the discipline that I mentioned before. That would certainly be a worthy reason. Having stopped Daf Yomi six years ago after my last Siyum (I was on my own schedule at times), I can certainly attest to the fact that while it is difficult at times to stick to the schedule, it is motivating to know that you always have a set piece of study that you must engage in each day. Sometimes, having a less-defined course of study makes it easier to skip a day now and again.
But whatever the reason, I think that it behooves all those who are serious enough about their learning to do Daf Yomi to ask themselves if they can push themselves a bit further. Face it, doing a second cycle of Daf Yomi barely counts as "chazara" (review) insofar as one's learning of each page is separated by seven and a half years for his last learning of that page. Perhaps incorporate some degree of real review into this cycle. Perhaps add another commentary to your study. Perhaps commit yourself to really read through the notes in the Artscroll or all of the additions in the new Koren/Steinsaltz or subscribe to and read one of the many Internet-based series of insights to the daily Daf.
Or perhaps even move away from the Daf. Choose a masechet (tractate of Talmud) and learn through it slowly and deliberately with the depth that Daf Yomi's pace does not allow for. Explore another area of the vast sea of Torah that you are not sufficiently acquainted with, be it Tanach or Halacha or Jewish thought. True, there are no international celebrations for any of these other schedules of learning, but the goal is for each one of us to raise our own levels of learning to the maximum that we have been granted the capability to achieve. On a communal level, I hope that Daf Yomi continues to provide the steady and constant opportunities for Torah learning that serves as its backbone. But on an individual level, I hope that each one of us has the honesty and sense of self to inquire of ourselves whether or not there are broader vistas that we should be exploring as well.