Thank you all for the responses so far!
I have not, but will enroll this October. I'm choosing Jepp though I live abroad currently so shipping the materials is going to be costly, so I'm just weighing the various materials outside of courses.
It's helpful to hear people saying 6 months out is too much time, I hadn't even considered forgetting knowledge as the test mostly seems to be useless info you soon forget anyways. Guess I'll give it a crack about 3 months out and see how I do!
If you can get a head start, do so. If you're presently working, you'll need the time. Face it: at the end of the day, you're not going to have a lot of time or energy, so I don't think that six months out is too soon to get started. If at all possible, try to get the exam out of the way before school starts; this way, you won't have that monkey on your back. If that isn't possible, then you want to be ready to take it once you get to school. Yes, if you start even three months out, you'll forget stuff. HOWEVER! However, it's much easier to review something and have it come back to you vs. learning it for the first time.
What I did was quit my job, and I studied four hours a day going over Sheffield's materials; I spent the nine weeks prior to school doing this. When I got there, I had to review it again, as I'd forgotten some of the things I'd studied early on. I got through all the Sheffield pdfs before I got there. Once there, I spent an hour a day on review of ADX material, and I spent the other three to four hours studying class material. They started offering the ADX late in week 2 or early in week three. I signed up for one of the week 3 exams, and I got it out of the way. FWIW, I got 91%. While it only takes a 70 to pass, I'd rather nail it with a comfortable margin.
While many advocate Sheppard Air; while that has its place; it's mainly good for the rote material; it wasn't so good for subjects that are better learned when they're understood. For example, FAR questions, which are rote, are fine with Sheppard Air; I don't know if I'd say the same for understanding weather, approach plates, and so on. I found that, if I really understood something, then I could reason or find my way to the correct answer; this is helpful when freezing up or having a mental block. By being able to REASON your way to the correct answer, you'll be able to blow through any mental blocks you encounter-a great feeling when you're racing against the clock!
Another reason why you want to try to understand as much as you can vs. learning by rote is that you don't know what questions you'll get; it's like Forrest Gump said. The FAA has a pool of what, 1,500 questions? Out of that pool, you'll get 70 of them. On my exam, I got a lot of pallet questions, so you can bet your bippy I was glad that I UNDERSTOOD the process of finding those answers! A good 10% of my ADX was comprised of pallet questions, so the preparation paid off. Your exam could be different, so it pays to prepare for everything.
Another reason I'm glad I took my time and really learned the material is this: it helped me in class! For example, the ADX has a lot of aircraft performance graphs and charts. What you'll notice is that each aircraft manufacturer has their own way of doing these. IOW, Boeing does their graphs one way; McDonnell-Douglas has their way; Cessna has a different way of presenting performance data; and so on. IOW, once you understand one Boeing graph, the rest will be easy. IIRC, the ADX had 737 graphs. At Sheffield, they were using the 727 for the class. Guess what? When we got to the 727 performance graphs, learning them was easy, since they were done like those of the 737! The lines and numbers were different, but they were laid out the same way. When we got to that part of class, I wasn't seeing this material for the first time, so I was glad that I'd prepared well.
So yeah, I understand what people say about starting your studies too early; I experienced it. However, if you're still working and will continue to do so until right before you leave for school, you don't want to start studying too late, either; if you do, you'll be behind the 8 ball once you get to school. Ideally, you want to take the ADX BEFORE school starts! If you can't, then at least get through all of your ADX study material before you go; this way, you'll only have to spend an hour a day reviewing before you take the test at Jepps. Finally, if your situation permits, I'd encourage you to learn and understand as much of the material as you can. Not only will this help you find the correct answer during the test; it'll help you in class too. Those are my thoughts. I hope this helps. I wish you the BEST of luck!