I have such a love-hate relationship with emoticons. On the one hand, they're harmless little icons that add a bit of cuteness, and on the other hand, they're tools of passive aggressive communication.
Overly dramatic? Well, have you ever gotten a negative or critical sounding IM from a colleague or friend. . . punctuated with a happy face? Sometimes people use them to take the weight off of a potentially negative message, but in my experience, it almost always backfires and makes the message seem even worse, like you're smugly smirking when you're saying it.
My advice for using emoticons is to only use them if they fit the tone you're already using. Don't use them to change your tone, because it probably will — but not in the way you want it to.
Think I take emoticons too seriously? I'm not the only one — there have been studies that show that emoticons mean different things to different cultures.