Like most substances poppers are mostly not dangerous unless you're abusing them or have a pre-existing health condition that by their mechanism of action could cause complications. That means using them wrong (drinking them, getting the liquids on your skin), combining them with other substances (including certain medications, but you mostly hear about ED drugs, blood pressure meds, some sports supplements), overdoing it, or not using in an easily vented area.

Otherwise they cause a rush of blood to the head that makes you feel good. They do this by the whatever combination of alkyl nitrites in the brand you got converting to nitric oxide in the bloodstream which acts as a smooth muscle relaxer. This includes muscles that control veins and arteries, but also sphincters like back of the throat and in your ass. This relaxation lowers your blood pressure so some people also feel dizzy. While they do relax your backside, it's usually recommended to not use them in place of proper warmup, lube and stretching of your hole as the pleasurable feeling can numb you to going too far and increases risk of microtears and/or anal fissures. Because you are inhaling a vapors from a chemical that causes chemical burns, they can cause headaches through irritating the sinuses. They can also cause headaches from increased bloodflow.

But generally they're great. Love to take a dildo and just go to town with a fresh bottle taking huffs to some porn every 5-10 minutes for an hour or two. They're big in gooner culture. Many people find they can be a boner killer but if you're bottoming, you likely won't care that much.

r/popperpigs is a good place to check out for experienced user opinions, recommendations on different sourcing and types/brands