I really love both Dr. Nebel and BFSU.
I understand the common hesitation at the book...it is not open-and-go particularly at first glance...but I feel it is a very solid approach that is at the same time rigorous and approachable for little kids. I would 'subtitle' the book (tongue in check) Avoiding Scientific Misunderstandings about the World for Beginning Scientists...I feel it offers just what it advertises, a fantastic Foundation upon which later study of science will be that much stronger having cleared up things commonly misunderstood.
I have been using it with DD since she was 4.5 (she is almost 5 now.) We are about 1/4 of the way through, so I feel like we're over an honeymoon period and still going strong.
I use it as a guide to conversation, more than anything else. My goal when we go through a unit is that when we finish, she will feel comfortable with the objectives listed in the front of every chapter. Some of these are very intuitive for her, so the lesson is more a brief reminder/illustration. Some take more investigation for her to be convinced.
I implement it roughly like this...I go through the book 6 or 8 lessons at a time and make note cards on each chapter; I copy the objectives, make notes about any activity I have in mind to demonstrate them, and I check the chapter bibliography against the local library catalog and pick out a book or many two on the subject. Then I file the cards in a box.
Once a week on Science Day, we talk about the new chapter and use the objectives as a kind of discussion guide. We often do an experiment in the book , but we've skipped them if DD can predict the results easily, too. We add a book about it into the bedtime story rotation for the week.
When she feels confident with the lesson objectives, she's been making a little science journal and recording/illustrating them as we go along. I keep this very simple. For example, lesson 1 is 'sorting things into categories' so our journal work for week one was getting the notebook and using stick-on tabs to organize it into the 4 science disciplines Life Science, Earth and Space, etc. She's drawn pictures of solids/liquids/gasses, drawn the earth and the sun and described night and day in a sentence, drawn different types of energy, drawn a food chain and a life cycle and labeled it etc...you get the idea. Most (but not all) of the chapter objectives get recorded into the journal and I catch her looking through it and reading the past pages we've made so I love this, too, it's clearly reinforcing the things we've discussed.
(I could post a few of her pages...but maybe just because I'm proud of them ;-) I've thought of making a blog for our BFSU because I enjoy it so much.)
To answer your question: I do not feel it's too much. If you tried to read aloud every word or read every book or do every demonstration it might be. But the ideas are not, in my opinion or experience, too much for a K'er (say, for example, the sun is on one side of the sky in the morning and the other at night...did the sun move? No, the Earth turned!) and how your child learns those ideas best is the flexibility that the book offers. Nebel is thorough and wants parents to feel they have a solid foundation, but IMHO does not intend to overwhelm small children. Just arm you to guide them through these discoveries about basic ideas of science.