TL:DR....I took a fatal dose and somehow survived.
I almost CTB with this method on accident while trying a different method. My parents had just kicked me out of the house over my drug use (which was pretty heavy at the time), and I was on the streets with no money and a bottle of 40 Vicodin pills (500 MG of Tylenol in each one). I wandered around a local park for a few hours before realizing that I could never do the homeless life. I decided to swallow all 40 pills and check out, thinking that the massive amount of hydrocodone would surely kill me. I didn't even consider that the Tylenol might.
I waited about an hour and didn't feel anything. Nothing. Not even the normal euphoria a bunch of Vicodin was supposed to provide. I gave up and figured that I had failed somehow. I called 911 and told them that I was severely suicidal and needed help. I didn't tell them about my attempt because I didn't actually want help. I just knew that I would be taken to a facility and not have to sleep on the streets.
Anyway, the cops came and took me to an ER. From there, I was sent to a county mental health place. All was fine, until about 24 hours after I had swallowed the pills. I suddenly felt a wave of severe nausea and ran to the bathroom where I proceeded to puke my guts out. I was completely drained of all energy and felt like I had a really bad case of the flu. I collapsed on the bathroom floor and laid there for a few minutes, until the roomie they paired me with discovered me and started screaming for help.
Next thing I know, I was in an ambulance being rushed to the ER. I didn't even think to tell them about the pills since I had taken them a full day ago. I was pretty lucid throughout the ordeal. The nausea was the worst part. There was also some stomach pain and, as I said before, a generally flu-like feeling. I was worked on by a team of several doctors. At one point, I remember two doctors looking at a computer screen, pointing to it with a look of shocked awe on their faces, then looking back at me.
Next thing I knew, I was waking up in a hospital room with an orderly from the mental health place sitting next to me. No one ever told me it was a Tylenol overdose or even gave me a possible cause. I only figured it out a couple years later when I randomly read an article about acetaminophen overdose. I'm still not sure how I survived it since, according to the article, both the dose and the length of time before treatment meant that I should have been a goner. I also found out why I didn't feel any effect from the hydrocodone. Apparently the huge amount of Tylenol overwhelms the liver, and it becomes incapable of processing anything else.
So yeah, it was an extremely shitty experience, but not "painful," per se. The kind of pain that scares me is sharp, stabbing or burning pain. This was not that. This was definitely unpleasant--if you've ever had a bad case of food poisoning, you're pretty much there--but definitely not the scariest thing I could think of. As a method, I'd have to rate it a 3/10. Not as effective as advertised, as evidenced by the fact that I'm still here. Not extremely painful, but far from painless. Still, I'm probably an outlier, and I could see how this might be some people's worst nightmare.