I definitely can tell a marked difference in vapor quality and quantity using the differing voltages, but a freshly charged batt has more "oomph" than one that's close to requiring a charge (not surprising). Having the choice of voltages allows me to get the vapor and taste I want from different combos of cartos and juices. Some juices taste a lot different at 3.3v than they do at 4.2v. I've tried using a higher voltage when the batt charge is running low to compensate, but a juice that tastes good around 3.3v but not at 4.2v still doesn't taste good if I up the voltage to compensate.
Oh, and the VV batts I use do not have a passthrough feature. They are these batts:
eGo 900mah Variable Voltage 3 Stage 510 Battery
I hope I made some sense?

All in all, I am really digging the eGo "type" VV batts I have; the only improvement I'd like to see is a shut-off mode.