Dr. Jensen is very well known in the research community, when it comes to caffeine-related studies. I remember reading some of his studies in graduate school, when I was helping my mentor (Dr. Jeff Stout) with some of his studies involving beta-alanine and creatine on lactate, ventilatory and anaerobic threshold.
I think caffeine is a great supplement in both fitness enthusiasts and endurance athletes. There are many health benefits to coffee and tea so I would recommend naturally-occurring caffeine drinks over carbonated drinks and energy drinks. Research suggests around 3-9mg per kg body weight, around 45 min prior to exercise. Most importantly, more is not better when it comes to additional ergogenic benefits with 2+ cups of coffee prior to workouts/racing. Although I believe that many endurance athletes receive a great boost from coke during a long distance event (although, I've never used coke in my Ironman races, many of my athletes have and really enjoy it), I stress the importance of paying attention to hourly calorie and fluid intake while drinking carbonated beverages. Because carbonation may bring on a feeling of fullness, it is important to pay attention to your liquid calories in an effort to receive the necessary electrolytes, calories, fluids and optional protein, needed to sustain effort and ensure efficient usage of fuels.
As you may or may not know, I have been a member of the International Society of Sports Nutrition (ISSN) for the past 6 years and I am certified by the ISSN in Sports Nutrition. I absolutely love being part of this society and having the opportunity to surround myself with so many knowledgeable people. You may not realize that there is often a lot of research behind many sports supplements (although some research is not validated nor scientific) and the PhD's in the ISSN are likely the ones creating the research. I have had the opportunity to meet so many amazing people such as Dr. Harris, who was a major pioneer in creatine research as well as Dr. John Ivy, who is behind Wheaties Fuel and the carb to protein 4:1 ratio. I can't even name the dozen of other PhD's in the society that have amazing research studies behind their names.
Here is an abstract from the ISSN regarding caffeine supplementation Goldstein ER, Ziegenfuss T, Kalman D, et. al. International society of sports nutrition position stand: caffeine and performance. J Int Soc Sports Nutr. 7(1):5, 2010.
ABSTRACT: Position Statement: The position of The Society regarding caffeine supplementation and sport performance is summarized by the following seven points: 1.) Caffeine is effective for enhancing sport performance in trained athletes when consumed in low-to-moderate dosages (~3-6 mg/kg) and overall does not result in further enhancement in performance when consumed in higher dosages (>/= 9 mg/kg). 2.) Caffeine exerts a greater ergogenic effect when consumed in an anhydrous state as compared to coffee. 3.) It has been shown that caffeine can enhance vigilance during bouts of extended exhaustive exercise, as well as periods of sustained sleep deprivation. 4.) Caffeine is ergogenic for sustained maximal endurance exercise, and has been shown to be highly effective for time-trial performance. 5.) Caffeine supplementation is beneficial for high-intensity exercise, including team sports such as soccer and rugby, both of which are categorized by intermittent activity within a period of prolonged duration. 6.) The literature is equivocal when considering the effects of caffeine supplementation on strength-power performance, and additional research in this area is warranted. 7.) The scientific literature does not support caffeine-induced diuresis during exercise, or any harmful change in fluid balance that would negatively affect performance