Dr. Mercola is a health and good diet advocate and businessman on the fringes of the anti-aging and healthy life extension communities. If you take a look at his latest newsletter (entitled "Popular Anti-Aging Techniques: Find Out Which Ones Work and Which Ones Don't"), you'll get a good idea as to his biases, opinions and the way in which he makes money from his business.
Dr. Mercola strikes me as one of the more responsible "better motor oil for human beings" businessmen out there. He is up front and well informed about calorie restriction, takes the high road on human growth hormone (don't use it), and is at least selling healthy diet information with scientific backing and no claims of a quick fix - even if it may or may not perform as well as claimed.
A good diet will mean you'll be healthier for longer - but it won't mean life extension unless you're practicing calorie restriction. Let me explain this comment, since this is a subtle distinction that is often lost. We all have some theoretical best possible life span, which we'll need to be in good shape to reach. A bad diet and lifestyle really means that you're reducing your actual life span by damaging yourself. A good diet and lifestyle simply avoids this reduction - but it doesn't lengthen your best possible life span.
The ultimate purpose of healthy life extension is to greatly lengthen the best possible life span you could have - not just to help you be healthier for longer within your normal life span. Currently, the only available technique proven to extend the best possible life span at all in animals (and with strong evidence pointing to the same modest results in humans) is calorie restriction.
Within this context, you might also want to read my comments on the meaning of anti-aging at the Longevity Meme. There is much the same difference between fixing age-related conditions and intervening in the aging process as there is between a good diet and calorie restriction. Remember: living a healthier normal life span versus extending the best possible life span.
Unfortunately, this distinction in rarely in evidence when people are trying to sell you things. So it's up to you to do the research and find out what is going on before you pay for products.