I've had some problems with mine. First one I bought, the connection point was a little too low and none of my clearos or my pro-tank could make a connection. Back to the store (fortunately I have a brick and mortar around the corner that deals in ecig supplies) and brought home the second. This worked well for three days on the initial charge, went to recharge it for the first time and found that it wouldn't recharge. Back to the store for another exchange, this time we checked that everything worked before leaving the store and I've gotten great results in voltage consistancy and battery life. I've verified each of the voltage settings with a multi-meter and been within .03v at every setting. One complaint I have that was consistent with all three units is that the firing button sticks from time to time. The battery does have a ten second auto shutoff which helps with that (would be better if the button didn't stick in the first place...) but that ten seconds before it kicks in is more than enough time to pop an atty, which I have had happen twice now. I've learned to quickly unscrew my tank when the button doesn't unstick fast enough.
Overall, a great way to get into VV vaping cheaply and having both a 510 and an ego connection means you can use just about every tank/carto/clearo out there. The pass-through feature lets you get by without needing a backup or spare batteries charging. The trick for me was getting one where everything worked like it was supposed to.