For reference, I used to use e-cigs, and eventually wound up working within the industry.
Please, please reconsider.
E-cigarettes are completely unregulated. Many of the juices contain substances that are known to be highly toxic. And before you tell me, "well so are cigarettes," some of these substances can actually damage your lungs
a lot faster and more severely than smoking does. Look up diacetyl and "popcorn lung." That stuff's in a lot of e-juices.
In addition, consider the parts within the e-cig itself. You're inhaling directly off of a hot coil. What's that coil made of? Yeah, I know what they tell you it's made of, but it's produced in Shenhzen, which is notorious for making dangerously sub-par Chinese products, and even if the nichrome they use is actually pure (doubtful) the material itself is possibly unsafe for this application. You can't avoid them. Even if you buy an American-made mod, you're still using Shenzhen-made atomizer or cartomizer.
In the case of an e-cig like the V2, which uses cartomizers, you're also inhaling off of heated polyfill. Polyfill that is very susceptible to burning, because it is wrapped around the heating element. If you don't believe me, I've attached a picture of one of my own cartomizers. Keep in mind: I did this for a living. I used absolute best practices, kept the cartomizers wet, rotated them regularly, and they still burned. I inhaled that.
The first picture is the burnt cartomizer filler, once I removed and unrolled it.
The second picture is the coil literally bursting into flames when I activated it.
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Beyond all of that, Smok and V2 are dramatically over-priced. They're nothing but a dressed-up auto KR808D-1's (the Smok looks like a mini battery - it'll be dead within 2 hours of moderate use). Auto's suck. They're unreliable and frustrating. You can get an unbranded kit with a manual switch for probably a quarter of what they're asking, and from a better-reputed website.
But my advice is that you stay away from them all together. Don't be foolish enough to believe you'll be able to tell what's safe and what isn't. I research everything absolutely to death, and I couldn't tell exactly how bad it was until I was inside the industry and seeing it first-hand.
A lot of this stuff is not out there for people to see, or so hidden that only months of involving yourself with it will give you any kind of clue. The industry is TOTALLY unregulated. TOTALLY. They could sell you arsenic if they wanted to, and they'd be accountable to no one until you were already dead. OSHA doesn't even know what they're looking for, on the rare occasion they care enough to try.
I have seen businesses sell juice using tainted nicotine that was so bad it was making the people who prepared it sick. Even after the owner noticed this, he still continued to have it shipped out. This was a company selling American-made juice, by the way.
I've seen them stuff down lab results that are very damning towards e-cigs. I've seen them just straight-up lie to people.
There are good people in the industry as well, but most of them are the really small shops, who are kept pretty much in the dark.
If you're curious, I eventually wound up getting so fed up with it that I whistle-blew, and of course, was subsequently fired. Regrettably, the sheer stress of that situation also resulted in me going back to smoking (I had quit vaping, due to all the things I just listed above). There is also an on-going lawsuit, spearheaded by my former supervisor.