I almost always wear only eyeliner on my eyes. IMO it makes more of a difference than eyeshadow or mascara alone, and it is MUCH easier to wear especially if you wear contacts or have sensitive eyes. I just use a pencil in a neutral color (black, brown, pewter, navy) under the bottom lashes only, it is quick to apply and if it smudges I can just wipe away the smudges with my finger during the day and reapply, no raccoon eyes requiring eye makeup remover. I also don't get irritating flecks in my eyes like with mascara and some eyeshadows. I would not leave my house without my eyeliner (and lipstick and concealer, those are my 3).
I do wear mascara every once in awhile, but most days I just don't because it is just so irritating sometimes and it is difficult to fix smudges and I can't rub my eyes all day then. I have dark eyelashes though, so all it really does is define them a little more, not really a big deal for me.