The single best study aid for me was First Aid For Step I. I believe that if you know the contents of this book, you know most of the material that will be encountered in the test. However, this book does not explain the concepts behind the information. It is assumed that one learned the concepts in medical school. Thus it is good if you already feel comfortable with the theory (ex. how the CV works, V/Q ratios and their significance, etc.) and you just want a source to give you a focus on what you need to study.
I did take the Kaplan class and some of the teachers were really good. I believe it did help me get a higher score by clarifying some of the subjects I did not understand. However, a lot of the "tricks" that they taught can be found in first aid. I do NOT recommend the Kaplan books for studying. They are way too detailed!
I also answered 90% of the questions on Q-bank, and read the answers on the ones I did not know, and sometimes on the ones I did. However, I thought Q-bank was harder than the real test and I got a 68%, so don't get discouraged.
I also answered all the questions in the Kaplan Q book. I like the majority of these questions, I do recommend this book.
I also did some questions from WebPath, but I'm not sure how much this helped me. If you have extra time, it's a good thing, but not if you are in a time crunch.
I had A LOT of pharmacology on my test, which was good for me because that was one of my strong points. There were also a fair number of molecular biology and molecular biology techniques. There were really no straight path questions, they were almost all pathophysiology. Know your cAMP, etc.
If you have prepared well, you will find that the test is in no way overwhelming. I found most of the test to be rather fair. I was dissappointed by two things, however. First, compared to the amount of material one learns in medical school, step 1 tests a very small proportion of that. Second, about 10% of the questions will be things you have never seen, and they will not necessarily be things that you can reason through. I had a lot of skin questions on my test, and very little skin pathology was emphasized in my education. For example, I had one question were they showed the face of a young man with mild acne, and they talked about him having some kind of a neurological condition (although not clear like I have wrote). Then they asked what drug was he taking. The answer was Lithium, it can cause acne. I thought this was a subtle point but I guess they didn't.
One other thing I recommend, go see the site before your test date. Ask them were the bathroom is, where the lockers are, check out the temp. to see if you need a sweater and make sure you know the way to the center. Make sure you have enough gas in your car. Take 2-3 powerbars with you and a couple of bottles of water, plus some healthy snacks and some light food. I did 2 blocks at a time and then I would take a short break to use the restroom and eat something and drink to keep my energy up. Also, if you are very nervous the week before the exam, don't hesitate to see a doctor to give you some low dose benzos, they can make a world of difference.
Let me know if you have any other questions.