I'm no expert but I'd imagine it's similar to getting put under for surgery, the drugs are similar at least, and the most painful part of that is the needle. Once the IV is in, there's no pain, just a slow and warm dark moving over everything.
I watched my mom slowly lose all faculty as she was reduced to a muttering, unintelligible, bed ridden shell. Over the course of a month she lost the ability to move all of her limbs except her left hand. She quite abruptly began having trouble finding and forming the right words and I never got to have the final conversations I wanted to, I was going to interview her about her life but one day I came back and it was already too late. She lied in that bed for weeks, stuck in the now permanent reality that she was going to die, slowly, as everything was ripped away from her one by one. The cancer took over the rest of her body. Nobody knew it was going to happen, we all thought she'd come home and be okay but she never walked again. It was distressing in a way I cannot communicate.
She deserved the warm dark, and it had been far too long by the time it finally came for her. One needle prick is nothing compared to the potential suffering at the end of life afforded by our current medical advancements.