Painful... hell yes. Dangerous? Well, as mentioned above all effects are temporary. Though some of them (stomach pain, frequent burning shits the next day) you will wish didn't occur.
But let me just say... I was diagnosed with irritable bowel syndrome, and I would take the stomach pain from too much hot pepper over the stomach pain from my IBS any day. They both suck major ass, but I wouldn't hesitate to take the pepper pain if I had the choice (even if it's more likely to be accompanied with burning shit/diarrhea). It tends to be painful in a different way, somewhat more tolerable, but doesn't last near as long. Luckily, my IBS stomach pain only occurs once every several months. I've nearly eliminated high fructose corn syrup from my diet (used to be a caffeine fiend and drink several 12-packs of Mountain Dew LiveWire a week
), and although it hasn't eliminated the IBS symptoms, it has made them less painful overall.
On an episode of Mythbusters ("Hot Chili Cures"), when asked if you could "die from eating a chile that's too hot," Dave DeWitt says that potential exists that you could possibly die of suffocation because your lungs would just shut down if you ate too much of something extremely hot. And I would imagine the mind-blowing pain and paranoia could possibly cause a heart attack or something just out of shock, who knows, but don't take that one as fact... it's just a guess on my part.
Now... I'll admit that I've never ate a whole super-hot pepper before (barely even a sliver of one... yeah, I'm a pussy when it comes to eating hot peppers whole and I freely admit it...). But habaneros tend to take my breath away (literally) when I eat them whole and the pain extends all the way up to my ears, so I wouldn't be surprised if it's at least somewhat true (though unlikely).