I know it's illegal in Australia, but I think it's okay pretty much anywhere else. You're supposed to be 18 before you can buy it in the US, though. Or at least in the states around here.
Salvia's a type of sage, and is commonly used for inner reflection and stuff. You can search around to find its history and stuff, especially at Erowid. From what I know (which isn't a whole lot, I am not an expert at all), there aren't really any negative health effects. Except, you know, inhaling burning stuff and all.
From experiences I've heard from other people, it seems that people either do it right and totally freak out (at least on the first time), or they do it wrong and it doesn't do anything at all. It's best to smoke it out of a waterpipe, using a butane lighter. You should burn it the whole time you're inhaling, and hold the smoke in your lungs for a while. It comes at you pretty fast.