I use the cone for looks and tightening the draw on my boge cartos. They have a tendency to be rather.... tight. If you are screwing on the cone and it's resisting, don't force it, or you may cross
thread it and never get it back off without cutting it off. With one of my cones I have to put the cone on first and shove the carto in.
I do like how much tighter it makes my draw, though. I tend to think it creates more visible vapor, because you're inhaling a greater concentration of vapor. The way I look at it; it's vaporizing the same amount of liquid no matter what, but the more fresh air you inhale with the vapor the more it will diminish the visible amount of vapor.
I can't stand the look of the eGo without the cone, though. Drives me nuts. That's why I can't get Rivas; they look awesome on their own (especially that spiffy blue LED) but they are glossy black and my eGo-T atties and 510 attie/carto cones are matte black. My brother's running a Riva since they're cheaper and that color difference is driving me insane.
Edit: My boge cartos are 2ohm (classified as low resistance) and, while they do get a tad warm, I don't think it's doing any harm.