"The difference between medicine and poison is dose." ME/CFS patients often require lower doses of medications than healthy people, and caffeine is no exception. Coffee and tea in amounts smaller than the typical amount in the American diet, i.e. 1/2 cup of coffee twice a day on more active days can be quite useful (though it does not improve the underlying disease). Caffeine is a proven performance enhancer, particularly in endurance exercise. Coffee has one of the highest concentrations of antioxidants of foods in the American diet. It is also a helpful adjunct for nasal congestion, allergies, asthma, and depression. However, it has effects that can be negative in ME/CFS, such as decreasing blood flow to the brain and activating the sympathetic nervous system, already overactive in this disease, as well as interfering with sleep.
In my experience, small amounts of coffee, tea, or chocolate can significantly increase my energy, physical and mental, without causing a crash later on or side effects. It also helps with respiratory symptoms like coughing. But, if I consume the amount of coffee normal people drink, it is likely to turn on me- instead of getting more energy, it will make me tired and dull, get autonomic symptoms like dizziness, and I will experience a crash as it is wearing off followed by increased trouble sleeping and next-day malaise.
Caffeine is nothing terrible for ME/CFS. I would not worry about addiction. However, it is best avoided during a serious relapse- if you are in that state where you need complete bedrest, and you feel like you need to avoid light and sound, caffeine will only make it worse.