In the early days almost all e-cigs used automatic batteries activated by drawing on the atty. Now they are rare, except in cigalikes.
Automatic advantages
- No button to press.
- Experience arguably closer to smoking, at least from the point of view of the action of it.
Automatic disadvantages
- Switches and pressure sensors often unreliable - oversensitive ones may self fire, or alternatively might need very strong draws to activate.
- Switch may be damaged by e-liquid leak.
- Automatic batteries do not work with many modern advanced atty's and tanks - you are mostly limited to older style cartomizers.
- Very limited choice of automatics now, at least once you get beyond the cigalikes which are mostly automatic. Anything "advanced" or higher performance will not have an automatic switch.
As already said, I think most would agree that a manual switch gives you more control. I've read once a modder was talking about the possibility of a more advanced automatic, with variable power controlled by the strength of draw, but I don't know if it ever got off the ground.