Wet 'n' Wild Lipliner in #712 (AKA Willow) is the perfect neutral lipliner. I actually think it's a closer match to MAC's Spice Lipliner than #666. It's more neutral with less wine tones than #666. And you can't beat the price. This pencil retails for only $0.99. These pencils have really improved a lot too. I remember them being very hard in the past, but the pencil I just bought is very smooth and creamy.
Basically you can use this pencil in two ways. You can either fill the whole mouth in with it over some lip balm for the perfect nude lip, or you can use it just on the lipline for better definition or to avoid feathering. One trick to avoid having an obviously outlined mouth is not to use the pencil around the whole mouth. Simply use it where you need it. For example, I have a very defined lipline except around the bow of my my lip, so I just use it there. Then I fill in with lipstick. If my lipstick wears off before my liner, I'm not left with an obvious outline around my mouth. Although #712 is very natural looking, I still think a keen observer would be able to detect the outline if I weren't wearing lipstick. This is just something I'm paranoid about! Overall, terrific product. I'll keep buying.