Ahhh poppers, now that certainly takes me back.
Most of what you asked has already been answered but a bit of background as well. Prevelant in the 70's gay disco world of San Fransisco, Amyl Nitrate was discovered to give a temporary buzz when sniffed through a bottle, which was great for dancing to. The reason it became more associated with Gay men, was because it also gives a hightend sense of anything sexual due to the speeding up of the heart rate. It also relaxes the Spinchter muscle as someone else mentioned which made anal sex easier. However now a lot of straight couples use it as well during sex to get that instant buzz.
Due to a loophole in the UK law, Poppers (or aromas as they are normally called now) are still available and are still legal. It isn't Amyl nitrate anymore however, it has been replaced by an even more synthetic mixture called something nitrate, my mind forgets!
It isn't in the same category as speed as it isn't an amphetimine based drug and the effects are extremely short term, however the main side effects are a cracking headache on the come down which is enough to put people off (you get a buzz for 30 seconds and then the worst ice cream headache for 10 minutes afterwards!), irritation under the nose, looking red, sometimes even scabs because the liquid is strongly corrosive and repeated "sniffing" through the bottle can leave the residue under your nose long enough to result in a small burn. And finally the smell, trust me it aint a bed of roses and it only takes one time to have someone nudge your arm and you pour the bottle on your clothes to know that this smell is here to stay!
Thats an unbiased report for you, hope it helps