CookyDawg is right Jmacburgertownnumber1, with a variable voltage device, its a lot more fun. I think the small 510 batteries push out a voltage of around 2.5 volts. (I really don't know for sure) I have heard the bigger Ego style batteries do 3.7 volts. These devices are fixed volt devices. (you can find VV ego style batteries, but I have seen on youtube that they are not great at being true variable voltage devices, they drain the voltage as the battery dies) With a TRUE variable voltage device, you can literally dial your voltage in, to your preference. You can dial in any voltage from 1volt, to 2.5 volts, to 3.7 volts, to 4 volts, to 4.5 volts, to 5 volts, to 5.2 volts, to 5.5 volts, to 5.7 volts, to 6 volts... And everywhere in-between on most of them. Some that I've seen go as far as 7.5 volts. Ok, so, whats nice about this is that you can use different style and different resistance atomizers and/or cartomizers, at different voltages, and find your sweet spots. For instance, a 1.5ohm cartomizer is nice at 3.7 volts, but at 5 volts, you would probably blow the atty/carto up! lol. So, rather than buying a 3.7 fixed volt device, you can have your VV device, and collect different ohm cartomizers/attys, and experiment with which voltage you like, for each type of atty/carto resistance!

Its fun. I can't wait to get my VV device! Also, different juices, taste different, and better, at different voltages. Your favorite tobacco juice might be really fruity and sweet at 3.7 volts and too much vanilla with your 4 volt device. But when you pop it into a cartomizer on your variable voltage device, you can find out what voltage it tastes best at, to your taste buds. OK, you try it on 3.7 volts and its the same as your Kgo, so you go to 4 volts and it has too much vanilla, yuck. So you pop the voltage up to 4.5 volts, and its getting better, but not just right. You pop it up to 5.5 volts and it tastes burnt. You dial down to 5 volts and it tastes EXACTLY LIKE A MARLBOROUGH RED!! And your like WOW this is the BEST tobacco I've EVER tasted, its JUST right!! haha.

So, thats why VV devices rock.