When I was pregnant, I was concerned about whether or not I'd know if I was in labor. After I figured that one out, I was concerned about knowing when my milk would come in. And, once I knew it was there, I was wondering if I'd know when it let down. Spoiler alert: you know. The feeling of breastfeeding letdown is distinct and unmistakable, in my experience, and feels as if someone is using an electrical current to deflate your breasts (I apologize for any mental images that may be causing you).
Letdown is in no way comfortable, but it's not necessarily uncomfortable and it's completely and totally tolerable because you know the one thing in the world that you need to happen more than anything is about to happen: your milk is going to be released so that your kid can have a meal. Hopefully your baby, or your breast pump (or a towel or something) is nearby.
However, I don't assume that my experience with breastfeeding and letdown reflects everyone else's because, like motherhood itself, breastfeeding is different for different women. Every mom and every body varies, of course, so it would make sense that we'd feel and interpret the sensations in different ways. However, I can't help but notice there's a common themes in how eight women described their own letdown: relief.