They're FANTASTIC! I love mine. I can get any flavor, any strength, and I can plug it in! So cheap, so lightweight/small, no lighters, no hassle, and if someone wants to bum a cigarette, we can just share. A 5ml bottle of fluid was only about 5 bucks, and it's the equivalent of a carton.
The only thing you want to watch for is "overdosing". That's what we called it. I did it, my buddy did it, quite a few people I know have done it. Its not as harsh or as 'filling' while you're getting used to it, and you don't have the 'oh, I ran out of cigarette' visual. Meaning, you don't have to put it out. You can just smoke and smoke and oh man, I don't feel so good, why is my head spinning. It takes getting used to. But once you do it's awesome. $5 for a carton, any flavor combination you could ever hope for, less carcinogens. For me, that was enough said, hahaha!