Note: This discussion of the positives and negatives of the Dr. Bernstein's Diabetes Diet is apart from those of lower carb diets in general. People whose bodies are suited to low carb eating tend to feel better on them, be less hungry, and have positive health responses (lower triglycerides and blood glucose; lower blood pressure, higher HDL, among others).
PositivesBlood Sugar Control - Dr. Bernstein advocates that everyone strive for normal, nondiabetic blood glucose (around 85 mg - note that the current cutoff for prediabetes is a fasting blood glucose of 100 mg, and 126 for diabetes). He feels strongly that this will prevent many health problems. This is lower than is advocated by "mainstream" diabetes treatment, but Dr. Bernstein has good evidence for his position on this. Many people following Bernstein's plan have had success in achieving this goal.
Simplicity - There's no need to fiddle around - there's one plan, and it's the same every day and forever. You eat what you want within very clear guidelines.
Bernstein is a very good writer. The books have great explanations, even when talking about complex topics. Even though Bernstein's position is controversial, it is very clear and easy to understand. He knows what he's talking about, and communicates it well.
Motivational - Dr. Bernstein has been on his own diet since 1969. He is so dedicated, and has such sound reasoning behind it, that it is inspirational to read.
Very restricted - This is a very low carb diet, and is meant to be followed forever. Only motivated people will probably be able to stay on it. Diabetes can be a great motivator, but weight loss and health might not be. Further, without a lot of nutritional knowledge, it can be difficult to continually get enough of all the vitamins, minerals, and phytonutrients needed for optimal health. While supplements can take care of vitamins and minerals, getting a range of phytonutrients is difficult on a very low carb diet. (I personally find it gets much easier around 40-50 grams of carb per day.)
Lack of individual variation - I believe that each individual has an optimum carb level. Some bodies can simply tolerate more carbohydrate than others. People who are not diabetic, on average, can probably deal with more carb, and would probably take to the diet better if it had even a little more carbohydrate. On the other hand, if everyone tested their blood glucose at least once in awhile, they would know whether their blood glucose was truly normal and what kind of diet it would take to get them there if they aren't.
Low volume diet - Dr. Bernstein has found that in some people, bulky fibrous foods can cause a blood glucose rise from a reaction in the small intestine (apart from carbohydrate). Therefore, he advocates not eating too much vegetable at once, even if they are very low carb. While I don't doubt that this "Chinese Restaurant Effect", as he calls it, is true for some people, I personally have not found it to be the case (I am prediabetic and test my blood glucose), and am guessing I'm not alone, especially amoung non-diabetics.
Potential boredom - It will take considerable adjustment and creativity to keep from getting bored on this diet, but many have proved it can be done.