I'm actually researching this option as my plan B and I stumbled upon this thread. Based on what I've read, they make your blood pressure and pulse very low until your heart eventually stops beating completely. Sounds peaceful to me, but hard to say. It will definitely have an impact on your mental state, less blood being supplied to your brain would make your consciousness gradually fade away.
This probably won't be instant though, may take hours and you're likely to survive if you get found in time. It's also hard to estimate a reliable dose. I suppose every method is a lottery to some degree, but if you want quick and reliable then it might not be your best bet. I have my own reasons for considering this method, I'm more interested in dying than death itself. Doesn't mean I'd like to survive it though. My plan would be take some meto over a couple of days, grind a shitload of pills in a mortar, mix them with water and drink the suspension. And by shitload I mean "definitely way more than I've read about in the medical articles".
But that's just my plan B, we'll see how plan A goes. I do have SN as plan C, but this isn't really my preferred way of dying, it's just my last resort.