For its size, the volt, halo g6 or bloog are the most powerful and robust ecigs on the market until someone invents something better that's that compact.
The kr808D-1 units mentioned are all the same inside, just different branding. They are all 3.7v regulated, compared to the 510 which is 3.2v and even the ego at 3.2v you're getting a true 3.7v device crammed into a superking sized pv. Not bad at all.
If you want an automatic they're very robust automatics. I've been using mine for... must be close to 8 months now, still have my original two automatic batteries that get used almost every day and they are just as good as the day I got them. Haven't broken, don't go off to sound, still activates when I draw through it, still holds a decent charge and doesn't misfire or go off when it's not meant to.
They also come in manual if you don't want an automatic.
You can get 220 mAh or 320mAh from the batteries as opposed to 190 mAh to 280mAh on the standard 510 and larger 510, and likely a worse mAh on true superminis.
The main reason I suppose most people say don't get a cig sized ecig is because of the charge they hold because they're smaller. I mean, the ego, has to be the most reccommended starter battery ever. But it's 3.2v and the kr808D-1's are 3.7v. It'll perform a little stronger than an ego. The only downside is 650mAh on the ego vs 320mAh on the volt. If charge is the only problem, invest in a PCC and take it with you. You'll get more "puff" time than an ego with two volt batteries and a PCC.
There pretty much is nothing else to get for cig sized PVs. When the new kr808D-1's came along, they topped every other cig sized pv out there and are still the top right now. Anything else is lower mAh, lower voltage, bad cartomisers/atomisers with barely any filling space and so many other things that just don't make a complete PV.
You don't have to take my word for it, but if someone said I had to use cig sized ecigs for the rest of my life or I can't vape, today, I'd be using my volt while I'm waiting on someone to invent something better.