Some folks use them as a supplement, some use them as a replacement, and there are even vapers who have never smoked before in their lives.
Personally, I use mine as a replacement for cigarettes. I've been tobacco free for 8 months and I feel a lot better. There are literally thousands of flavors that one can vape in these things. Right now I'm vaping a liquid called Jamaican Ginger Mint. Really tasty. At first I thought I wanted something that tasted like a cigarette but then I realized that the key to quitting cigarettes is that you have to like vaping better than smoking. That's where the flavors come in to play. It tastes so much better than any cigarette I've ever had and I smoked for 36 years. You can even buy liquids with different levels of nicotine. Some people even vape zero nicotine. I recently stepped down from 18mg to 12mg. Maybe I will eventually get down to zero nicotine but I don't really plan on it.
My wife also gave up cigarettes for these. I was surprised at how easy it was for both of us to quit smoking. We could never do it before for any longer than a few days. Our doctor is even happy that we made the switch.