My batteries showed up yesterday, so I've been happily using the Mini VapeStack today. I'm using LiMN batteries, the AW IMR16430. After a day of usage (10 hours), it was still going strong, showing 3.7v on a multimeter, down from 4.2 fully charged. Amazingly putting out more power than a fully charged 510 batt after so much usage.
At last... no more constantly recharging the handful of mini/supermini ecig batts I've got lying around to eek out a little more performance
So far the 901 atty is my favorite atty for this device.
401 attys work better at lower voltage (best at around 3.4 if you ask me, really nice on a std 510 batt). But the 901 atty is much happier at these higher voltage levels, and a KR808 carto that really didn't produce much vapor on a std 510 batt works just fine with the extra power on the MOD. Haven't tried any 510 attys yet, but I've got a joye510 in the mail and two SLB510s on deck. I also want to try at least one penstyle atty (302 maybe), but I suspect those will be bigger than I prefer really.
I was a little worried the button placement would be awkward but it works well enough for me. I mostly hit it with my thumb while holding the mini in a loose fist with the bottom resting on my pinki finger. I haven't been using the drip tip since I fill carts instead. Also i just don't like the idea of a metal tip honestly (think cracked teeth when getting unexpectedly bumped).
Its got a real nice solid feel to it and I like the asymmetrical look.
I picked up a carrying case for it that works well with it from REI.
Keychains - Keychain Pouch | Marsupial Wallet Key Chain | Chums
I've replace the keyring with the smallest size S-Biner so the case can hook to a belt loop.
http://media.rei.com/media/ll/840b5c7e-ea2a-493f-8d5f-747c8ff1f8f9.jpg
The MOD slips into the outer pouch compartment (90mm tall) and the zippered compartment (105mm) is large enough to hold some combination of spare carts, small bottles, spare battery. The whole package easily fits into a pocket but I had it clipped to a belt loop today.
[edit: This arrangment is better than a separate mini altoid case clanking around in my pocket against the vapestack.]
I'm a happy camper

The Mini VapeStack is a definite keeper. I'm going to get a second battery MOD that takes 16340s too, and depending on which proves to be most convenient/comfortable to me, one will serve as a backup to the other.