Several people have mentioned the fact of where they are "appropriate" and such. I do agree that it is not professional to sign emails with lots of smiley faces. Personally, I use them a fair amount in talking (online really. I can't stand the ones in my text messages haha). But I also think it is how it is done.
If you are replying and after ever sentence it is
Then that is a problem.
If you are having a conversation and you throw in a few xD or
, then I don't see a problem with that.
I think they do help with context. "You are such a pain =P" is a light joke. "You are such a pain" sounds like they are really annoyed. It really helps considering there is no sarcasm font (although there really should be). I probably use smilies and "haha" too much. But I guess the same goes for "lol" and such.
Plus, how can you not use one like this?
I mean... it is just too cute hehe xD