I tend to run 4.5V with my Boge single coil 3 Ohm cartomizers. Though, I may crank it up a lot higher when I want to take short hits.
The vapor is just too hot with the multi-coils for my vaping style. I vape like smoking a tobacco pipe. What I want is a dual barrel setup for my cartomizers. That way, I get more vapor, not more heat. I can come up with a 1->2 connector, but I can't think of an easy way to make a 2->1 mouthpiece that doesn't look like bad plumbing. Putting two cartos in your mouth is really uncomfortable.