Long-term longitudinal studies show that coffee is good for you. They don’t actually show that caffeine is good for you, as far as I’ve seen. So as a health benefit, until we know exactly what’s doing it, you may want to drink real bean juice. (Teas are also good for you, and both coffee and tea have caffeine, but there are a ton of other substances in coffee and tea that are likely having positive effects… I suspect that caffeine, itself, in moderate doses, is also having a positive effect, but it’s hard to say how much of the benefit is the caffeine, versus other stuff in the coffee beans/tea leaves.)
Fortunately for me, I love drinking coffee. Also, the staining problem really only happens if you drink your coffee black. Even using a tiny amount of creamer - whether it be dairy or soy creamer or other - pretty much eliminates the staining effect.
I haven’t read into the effects on adenosine receptors, as far as i can recall, so can’t comment there… but I do know that caffeine loading for athletes is well-studied. Caffeine provides less and less of a performance-boosting effect for habitual users, and they would have to resort to a massive dose on competition days to get an effect, which comes with risks of side-effects and adverse impacts on performance. They’ve found, however, that stopping caffeine intake for just 4 days to a week before an event restores all or most of the performance-enhancing effect on event day. So the body seems to rebound in sensitivity to caffeine quite quickly. I don’t know if the mechanism relates to adenosine receptor response.