I am a "powergamer", by the definitions of one here- I know the necron codex by heart and can rattle off rules and point costs, and I always play to win. However, I'm not an asshat about it. I know I'll lose some and I don't try to cheat. In the 40+ games I've had I've lost a good share, possibly more than half, but I don't care. I jsut want to have fun, but knowign what you're doing makes it more fun because you don't have to fetch a rule book every ten minutes and you can just play your game. Rule conflicts pop up every now and then, such as fearless psykers too close to my Pariahs- What wins, Psychic Abomination or Fearless? But these conflicts don't have to be hostile, just look at the rules and make an intelligent decision. If you can't reach a conclusion, do a rolloff. Then go back to your game. Simple.
The point is, a powergamer is more a person who does nothing but play, and tries to win, not just a person who intimately knows what they are doing. Or maybe they are. Everyone has a different definition of what a powergamer is.