As you move up in power you get more warmth, vapor, throat hit.
For a given resistance atomizer/cartomizer, higher voltage = more power (and current draw).
For a given voltage, lower resistance = more power (and current draw).
Say you have a 2.5 ohm cartomizer. On an eGo at 3.3V you get 4.4 Watts, pretty cool. The same carto at 5V is 10W, at the hotter end.
Somewhere up around 8W is, for me, pretty nice. Easy to get there with a true passthrough or a decent variable voltage mod, but not so much on that nominal 3.3V eGo -- running a 1.5 ohm dual coil carto gets to 7.3W, plenty close enough, but you're drawing (or trying to draw) a 2.2 amps, not sure what long term effects it will have on a stock battery.