It's not painful in the traditional sense but you will get a bad headache, have to be putting up with a taste and after taste that makes you want to vomit, nausea, fast heart rate, high anxiety, possibly stomach pain (some people have reported this). It just sounds like a horrible experience. And yes, you can take ae and benzos and etc to make it more peaceful but you'll still feel these side effects to a certain degree.
I'm planning a complex CTB: swallow capsules of SN, use an exit bag and full suspension hang. I feel I covered all my bases here. My family also knows my wish to not be kept alive if I am brain damaged.