I grew up in west Texas. Plenty of rattlesnakes and copperheads to go around. I think every dog we ever had was bit at some point, usually in the head. They'd lay around for several days and act like they were dying, ears and head would swell up like a balloon, then they got better and knew to leave snakes alone after that.
I never got bit, but a knew a couple of people that did. They said it was very unpleasant. One guy got bit by a copperhead on the thumb and had to have it amputated because of infection (not the venom). Anyway, the snake bite kits are a gimmick. Folks that live in snakey country don't buy them. The common wisdom is don't go to cutting and sucking on it like they do in the movies, that has long been proven to be worthless and just wastes time. Just walk slowly so as not to get your heart rate up, keep breathing slow and deep, stay hydrated, and get to the nearest ER. Like an old farm dog, a healthy adult will usually survive a snake bite with rest and hydration, but its certainly not recommended.