+1 on Ken's Big Daddy.
Big Daddy uses the most recent type of switching regulator that maximizes the efficiency of battery life, so that two regular (not high-drain) 18650s---providing anywhere from 4800 to 7200 mAh---seem to last forever. Like nomoreash, I change out the batteries in my Big Daddy every 2-3 days and have never yet run them down to the point that the regulator's safety shut down kicks in.
While the Big Daddy doesn't have the sexy high-tech displays of the ProVari or Darwin, it's well-made and solid, simple to adjust voltage (just turn a little dial on top), and a heck of lot less expensive than either of those top-end VVs.
Big Daddy Variable
Ken's smaller two-battery VV, the Big Brother, uses two 17500 900mAh batteries and also has very impressive battery life. It's extremely compact (2.5 x 1.5 x .625"), almost a stealth vape, whereas the Big Daddy is necessarily a large box (about 3.9 x 1.9 x .9") to accommodate two 18650s.
A refurbished Version 1 Big Brother (I own two of those and love them) sells for $60 shipped. Heck of a deal.