I've been vaping for over a year and a half. I started at 24mg nic and am currently down to 12 mg nic. I use approximately 2 - 3 ml a day.
The eliquid contains a mixture of pg (propylene glycol) and vg (vegetable glycerin), liquid nicotine, and flavoring agents. As said above, no long-term scientific studies have been completed on the safety of inhaling this as vapor. What is known is there is no tar or smoke being inhaled, so no carcinogenic materials.
Nicotine can be considered a poison in the right amounts, but we aren't vaping that much in our e-cigs. Nicotine is also considered to be a vasoconstrictor, meaning it can cause the smaller blood vessels to become smaller. But so is caffeine, a very similar drug found in coffee, soda, and chocolate. There have been some recent scientific studies that show that nicotine may help prevent Alzheimer's Syndrome.