Rumor has it that Evolv, the same folks that do the Darwin, might come out with a screw on kicker button that'll work with any of the non regulated big battery mods. That'll be so cool if they do this and it can work with bottom feeders. I'd love to get my hands on a Variable Power REO
If I already had batteries and chargers laying around, I wouldn't hesitate to pick up a Buzz from notcigs as a backup for my Darwin. Only reason I haven't, is because I don't have batteries and chargers, and dread the idea of battery clutter to contend with on a daily basis.
The
V2 VVPV from notcigs looks very promising. It will have a built in charger
The main reason I love the Darwin (aside from the Variable Power Technology): I never have to mess with batteries and chargers

That's the whole point of it going Li-Poly! The battery technology is sooooooooooooooooo much better. Just top it off every now and then like you would a cell phone or lap-top. Li-Poly batteries paired like this can crank out up to 12v in far less space/weight than it takes to do with other battery types, and they typically have a life span of at least 2 years. So unless you get a duff battery.....it'll be a LONG time until you ever need to touch it. Price per mah and volt to you as a consumer...well, these are pennies on the dollar compared to the old Li-Ion or Li-Mn battery route. Can also vape this thing while it is charging.
Consider:
Having to carry a charger unit, a voltage meter, open the mod and CHANGE the battery at least once per day. Stow and carry all the bits...
VS
Just plugging the whole thing in at bed time every couple of days. Heck, since I sleep in a place with electricity I just plug it in to top off every night out of habit. The Darwin also vapes as a passthru battery...while it's charging.
As someone who travels a lot...it sure has been nice to just pick up a Darwin with a loaded tank, a little USB cable, and go all weekend on mondo road trips (USB chord isn't really needed unless the trip extends beyond 2 days, or I decide to pop a dual coil carto on it). The thing even rides nice and flat on the console or car seat beside me with the tank held upright (Arm set between 90 and 115 degrees).
The Darwin form factor is different, but it's really nice and flexible...easily pocketable with regular and eGo sized cartos/atties/tanks.