It was surprisingly good. The core idea can be summed-up with an old and simple phrase, "first things first". Despite being important and, we hope, a simplifying force, your goals in each area of life can get complicated. You get confused that some things can't be dropped (e.g. seeing your daughter's appearance in the school play), while other - equally important - things can (e.g. improving your presentation skills...at least for this week). Moreover, you get lost in the weeds and forget how action on any given day or week connects to the big, long-range goals.
The book tackles these challenges by focusing on just one question, "What can I do such that by doing it everything else gets easier or unnecessary?" Having considered the big "anytime" goals, you can relate those to a 5 year, 1 year, 1 month, 1 week, 1 day, and now-view of action. Thinking about the other temporal direction - i.e. extrapolating from current needs to the long term - is very difficult.
Next comes "time blocking". Here the book becomes a little arbitrary. You are told to lock yourself away for 4 hours per day, 5 days a week, to work on your One Thing. That's fine as it goes but, no doubt, some people could manage more or less than 4 hours, given other constraints...No matter. The book has lots of other tips but the real value is in producing a fairly sophisticated framework to justify the ultra-simple; finding ways to focus on the one most useful thing. I recommend you read the entire text.