I expect that Gabbey gets mediocre reviews because the majority of philosophy students find the subject interesting to an extent in itself, but rely on professors to make it magical. Gabbey won't be performing any magic. If you're the type of individual who can be made to love philosophy, and you're looking for teachers who will create that fascination in your upper level classes, I might recommend Vogt, the Kitchers, or Bell, among others. And indeed, I wouldn't recommend Gabbey. To repeat most of the reviews on here, he is not a dynamic teacher as "dynamic" is traditionally conceived. If that's what you're looking for, you will be bored nearly to death in this class, and while you'll probably still be able to pull off a decent grade if you're quite smart, you'll get relatively little out of the experience.
However, if you find philosophy to be extremely interesting in itself, and are taking this class with the priority of advancing your own knowledge, ideas, writing, and capacity to understand extraordinarily important and complex material, I would recommend Alan Gabbey more highly than any other professor. This man's understanding of early modern philosophy is unsurpassed. He is a scholar in the field, and he is more than willing to share his understandings with his students. You have the potential to gain as much as you want in this class.
Far too often, I find that Columbia philo professors aren't really listening to students' questions or genuinely responding to the concerns brought up in their papers. Professors often think they know what a student is asking before the student has asked it, for instance, and the professors often aren't quite right. They read papers closely enough to bring up general objections, but not closely enough to acknowledge that the student has already addressed those objections either explicitly or implicitly. I'm certainly not saying that the professors cannot understand what what the student is saying, and I'm sure that most of what the students are saying, given their levels, can be reduced to something completely basic or blatantly misguided. Furthermore, I'm clearly speaking from my own subjective experience. But nevertheless, it's rare that my deeper concerns are ever acknowledged or examined in a way that allows me to understand exactly WHY they're basic or misguided - so I rarely find that my concerns have been addressed in a way that renders them no longer concerns. I think alot of philo students feel this way, and I think that sucks. It's frustrating and discouraging, and sometimes leads me to feel like philosophy is less about pursuing truth than impressing others with seeming-brilliance.
Professor Gabbey is above these criticisms. When a question is asked in class, he does not try to reduce the question to something simpler before the student has fully explained what the question is. He does not give general objections, or focus on that which is most fascinating for the most students. He moves slowly and accurately through the material, assuming that you have read it yourself, and are frankly above needing to be fascinated by it. I've always felt that Gabbey teaches to those of us who are looking for the most complete understanding of of the material, rather than to those of us who are looking for the easiest or most interesting way to understand it. He stands against reduction especially. Do not think that you'll be able to come into this class and get away with saying that any parts of Spinoza, Liebniz, or Descartes are "basically the same." And if you ever do, you had better be prepared to defend yourself against a very deep, intricate, and involved explanation for why that stance is indefensible. This class is about learning intricacies and complexities, and that's rarely fast, fun, or easy when done right.
Additionally, if you're looking for generalized comments or "good job!" in response to your papers, look elsewhere. Gabbey will analyze your papers deeply, giving extremely thoughtful feedback and thinking very seriously about what will make you a better philosopher. Don't expect to be directly encouraged with kind comments, and don't expect to treated in a special way in class if you make lots of good comments; Gabbey answers and responds to all his students - both the ones who proved themselves to be quite good and the ones who proved themselves to be quite questionable - in the same ways. But this is not to say that Gabbey is not extremely kind! If you reach out to him, he will help you immensely. He will give you real help on papers both before and after you turn them in. Not only will he advise you about additional things to read in preparation for your specific paper topics if you let him know what the topic is, but he'll give you a solid amount of real feedback as well. He is also extremely nice about deadlines and absences, and isn't a terribly hard grader. You might receive a page an a half of typed comments outlining all the issues with your paper and still receive an A or A- on the work.
I think Gabbey most rewards ambitious philosophy, but also seriously criticizes ambition which fails to acknowledge the depth and complexity of the issues involved. And in turn, I found this class to empower both my philosophical ambitiousness and my understandings of the depths of metaphysics. Gabbey is certainly my favorite professor thus far, although I understand why many wouldn't regard him in this way. But for those who are genuinely interested in philosophy, I recommend Gabbey most highly. You can expect to learn from a true master in the field of early modern philosophy, and someone who takes a very quiet but very real interest in you. Gabbey will take you seriously as a philosopher, and he will take your philosophy seriously. In turn, if you take this class to be a philosopher in some sense, and that is your true motivating reason, I think you'll gain more from Gabbey than you could have possibly hoped. But if you're in it for any other reason, especially just to fill the requirement, then I most highly recommend you pursue another class with another professor.