Some interesting thoughts here already.
I think the main point of consideration for me right now isn't that I feel particularly heedless when consuming coffee, because I don't [more on this below]. The way caffeine works in the brain (my limited understanding of it) puts the substance into a similar category as other non-permitted stimulants. First, it stops one chemical (a Google search tells me this one is called adenosine) from binding to receptors in the brain, helping to prevent natural drowsiness, then it aids in the increase of another, dopamine, (this is the "feel good" chemical in the brain).
The fact that it works this way just gets me wondering about it, so I figured it was worth discussing as I enjoyed my morning coffee. Hope that sorta makes sense...
As for heedlessness on caffeine, I have consumed so much of it, and with such regularity, that while I do not feel like I loose my heedfulness while it is present, I have to admit that I am unable to clearly recall a time when I was sufficiently caffeine-free as to have a truly objective point of view on the matter.