Research recently reviewed the "growing body of data", analyzing the health effects of our collective favorite beverage. It appears that habitual coffee consumption is neutral to beneficial in its effects on various aspects of cardiovascular health, including coronary artery disease, congestive heart failure, arrhythmias, and stroke. Coffee also seems to help prevent neurological degenerative disease, asthma, and certain gastrointestinal diseases. The average consumption in these studies was 2-3 cups daily of the caffeinated brew.
It's a little bit of a mystery, isn't it? For many people, caffeine has an adverse health effect, either anxiety, insomnia, nervousness, or rapid heart beats. Decaff coffee has some of the observed benefits but not all.
The conclusions were qualified with the reminder, always so important, that almost all studies are observational and not truly tested in a scientific way. Meaning, it just might be that people who are healthier in all those areas, just happen to drink more coffee.
If you enjoy coffee, great, you can feel good about it - and I would say, decaff or not. Remember, too, that the other beverage, black or green tea, has also been shown to have numerous health benefits, so tea drinkers needn't switch!