Personally I don't tend to enjoy evil playthroughs simply because I don't like being mean to people who don't deserve it. I don't like kicking folks who are down, betraying people's trust, it just feels bad to me. The closest I've come is playing somewhat lawful evil tyrant types who think that in exchange for providing a baseline quality of life they as those of higher standard should be free to do as they wish beyond that. So I rarely play evil, at most I play harsher degrees of good.
Another thing is, I find that evil has less room for personal growth. Its all about what you can get externally. Its selfish and power hungry and any introspection you have can't lead to growth. Playing a good person corrupted to evil sounds like a downer, and playing someone who starts evil and stays evil leads to a flat character that seems boring for me to play.
