In informal communication (emails between friends, comments on message boards, tweets, etc), it's fine. Smiley away: they enhance clarity.
In formal communication (a work-related email, a scholarly paper, a letter to the editor, a published article, etc), it's not okay. The smiley will make you seem less credible, less mature, and less professional. In formal writing, your words should make your meaning clear. If your meaning might be misinterpreted, do a re-write until it /is/ clear.
Smileys are for your time off.