Almost the entire course focuses on First Aid, with a little supplemental material and constant quizzing. The interactive format helped solidify some main points, which was good for me as I tend to gloss over material sometimes during my studying. Jenkins points out which subjects are guaranteed to be tested, and which ones you shouldn't even waste your time with, and I didn't feel like he misguided my studying in any way.
I studied for 6 weeks total. I did not do the weekly question sets sent out by DIT. I gave myself a little time before the course to get through First Aid once and work slowly through major problem areas (biochem/neuro), as well as a week and a half afterwards to review weaker areas again and do the final 5-day First Aid review before taking the exam. I did about half of USMLE World, but my primary study sources were definitely First Aid and DIT.
My practice NBME right after final exams was a predicted 200, and my final score was >240. My previous performance background: high pass/honors in all courses in the first 2 years, and a high MCAT score. Just to echo others - there is no replacement for working hard in the first two years.
I really liked DIT because it kept me on schedule, while allowing me plenty of time during the day to review independently. I hadn't put a lot of thought into HOW I would study for Step 1, and I started to get freaked out when my friends showed me their carefully planned schedules, detailing practically every hour of every day. With DIT, I felt secure knowing that Jenkins had been there and coached many students through the prep period, so as long as I kept up with the course, I would be in good shape for a solid review of First Aid during those 3 weeks. Jenkins is completely correct in telling you to take the exam within 2 weeks after finishing. That last week before my test I was getting burned out and studying wasn't very productive.
If the descriptions in this thread make the course sound like something you would like, then I would definitely recommend it.