8 out of 10 studies with trained athletes show significant benefits of caffeine, on performance or physiological responses or both. However, it has to be used properly. Studies show that it stimulates the nervous system, increases the release of adrenaline, increases the use of body fat and spares glycogen.
Recent studies using athletes as subjects found no diuresis or thermogenesis...... they were found in sedentary folks. In other words....if you are highly trained caffeine is not a diuretic. Anecdotally I have found this to be so.
The most common beneficial finding of caffeine use is the increase in burning of bodyfat as fuel. As you all know, if your body ids burning more fat for energy it is sparing your muscle glycogen and therefore will extend the amount of time you can go hard in the gym or elsehwere.
However...you must take it at the right time. Fat burning does not begin for 3-4 hours after digesting caffeine. Taken during or right before a workout will probaly just make you hyperkinetic after your session.
Allow me to cite a study done by Dr. McNaughton at the Tasmanian Institute of Technology in Australia. Caffeine at 10mg/kilogram of bodyweight was given to athletes 3 hours prior to vigorous exercise. A dose of 750mg for a dude weighing 165lbs. Results showed that caffeine increased time toexhaustion by 18% and exercise intensity by 24%. It increased use of free fatty acids for fuel, thereby sparing glycogen. It also increased the lactate threshold in relation to workload, suggesting that there was less build up of lactic acid. If you use caffeiene correctly, this is the level of benefit you might expect.
Finally...best to use caffeine from coffee....not pills, cola or guarana. Columbian is da kine.