Function is #1 in my book. It's got to be reliable and VV. Once those are met "form" comes into play. By form I don't really mean looks, mainly size. It's got to be pocket friendly. My workplace basically treats vaping the same as smoking, so I've got to go outside to smoking areas or be on the stealthy side. My two favorites are Mini Provari with Ody on it and a mini SweetVape with a 306 on it. Both fit pocket and hand well enough that I can usually sneak a few hits in when I need them. Also they can take a big chain vaping session when the opportunity presents itself. I do sacrifice some battery life but its better than a mediocre allday vape. Extra batts are easy enough to keep around. Here's the mini SV, its a VV bottom feeder with a 3ml bottle in it:
