I'll be happy to share my opinions. First of all, there are different learning techniques that work better for some people than others. I doubt that there is any one best way to learn. I think you have to find what works best for you. Then too, a lot depends on your schedule and learning goals.
When I started Premium, I was able to go through it pretty quickly because I had some German in college as well as self study. I pretty much concentrated on just the interactive lessons. Of course, when I first started Rocket German, some of the features available now weren't available then. I worked my way through Premium Plus and just started Platinum when the new release became available. Then, because I wanted to score some points, I went back and started doing the other excercises, particularly the recordings. That was a real revelation to me. I was pretty good with the Premium level, but discovered that I still needed a lot of work with Premium Plus, not to mention Platinum.
So, I went back to do what I should have done in the first place. I redid every exercise of every lesson including all recordings, not just once, but until I could honestly rate them as
"Easy" or worst case"Pretty Good". I'm now about half way through my Premium Plus review.
I think all of the exercises are important. Grammar might not be the most fun, but it is important and the way the lessons are structured, the recording phrases in the grammar section supplement and reinforce the voculabulary you learned in the interactive lesson, not to mention what you learn about grammar. I've found it best to the do the interactive and grammar in parallel for that reason.
If you are at all like me, there will be a tendency to want to rush into the next lesson and bag some more points. The points are a marvelous motivation tool for those of us who are competitive. I have to work everyday to maintain my discipline and not move ahead until I've done every exercise as many times as necessary. As you go forward, the lessons get harder and the phrases longer and more complex. I write them down in long hand, over and over like a naughty child in grade school. It works for me. Using the vocabulary builder works for the same reason, i.e. the act of writting something down helps you remember it. When I enter a word in the vocabulary, I look up all of the definitions of the word and enter them, not just the meaning used in the translation. In all languages, I expect, German and English for sure, the meanings of words can change with the context of the sentence and topic.
I think you have to set goals and milestones for yourself, but that has to be based on what is reasonable for your schedule which might be several hours a day or perhaps only an hour or two a week. There are many good learning and study tips in one of the sections. They are well worth reading.
You need to keep the learning fun and fresh. Do the games and flash cards from time to time. Browse the web occasionally to check out material in your language. Try to read a book or newspaper.
Good luck all my fellow students and remember the words of Red Greene, "We're all in this together".