Deciding to use caffeine is an individual thing, but science shows that caffeine is beneficial as a performance enhancement tool as well as a fat loss tool:

Chemical Mechanism

Caffeine is a central nervous system stimulant and causes an increase in psychological and physiological arousal from the sympathetic nervous system.

As a result of this stimulation, the catecholamine epinephrine is released from the adrenal gland. Epinephrine may improve the inotropicity or contraction strength of the heart and increase the use of fatty acids as a fuel source during aerobic or endurance exercise.

Lastly, caffeine plays a role in the release of calcium from the sarcoplasmic reticulum storage sites in the cell. This may stimulate more powerful muscle contractions for anaerobic or power sports.

Contraindications

Persons with a history of heart disease, high blood pressure, or other various related health problems should consult their physician regarding caffeine use.

Precautions/Warnings

An excessive consumption of caffeine may vary from person to person depending on tolerance and past use. Possible side effects may include nervousness, shakiness, anxiety, heart palpitations, flushing, and anxiousness.

References

What caffeine can do for you—and to you. Consumers Union 1997;9:97-101.

Graham, T., and Spriet, L. Caffeine and exercise performance. Sports Science Exchange 1996;9(1):1-5.

Cole, K., et al. Effect of caffeine ingestion on perception of effort and subsequent work production. International Journal of Sport Nutrition 1996;6:14-23.

Anselme, F., et al. Caffeine increases maximal anaerobic power and blood lactate concentration. European Journal of Applied Physiology 1992;65:188-191.

Costill, D., et al. Effects of caffeine ingestion on metabolism and exercise performance. Medicine and Science in Sports 1978;10:155-158.