I didn't feel the transformative effects that many blog about, but I definitely felt how tired I _really_ was.
I started napping way more during the day than before, and I'd fall asleep within ten minutes or so on most nights once I hit the bed. (I had been doing CBTi for a week or two up to the time that I quit. I don't have sleep apnea.)
When I drink caffeinated coffee now (extremely rarely), I definitely feel it and definitely crash later in the day as a result. (Caffeine lasts about six hours in your system before its metabolized away.)
What I realized pretty quickly into doing this is that (a) caffeinated coffee is probably the only drug that is nearly universally socially acceptable because (b) nearly everyone is terribly sleep deprived.
It's a real shame that modern (Western) society is designed from the top-down to sweep mental and physical health under the rug in the name of "efficiency". Coffee's just one unfortunate side effect of that.
Also, it is really hard to get good decaf drip coffee but, surprisingly, much easier to get decaf espresso. I'm guessing this is because decaf versions of coffee-based drinks (lattes, flat whites, fraps) is more popular.