Dive summary:

• A recent trend in email marketing is the use of special characters in subject lines.  Examples: A heart for valentine's day, a sun for hot summer deals.
• The increase in usage may be due to a decrease in technical issues, making rendering much more reliable it than had been in the past
• There are reports of increased open rates due to the use of special characters, but you still have to make sure it makes sense for your company.  A special character can not rescue a bad subject line or offer.

From the article:

Around Valentine’s Day this year, I noticed a couple of organizations using a heart in their subject line.

Since then, I’ve seen more and more companies starting to use symbols in their subject lines in order to grab the attention of a reader. Using special characters in the subject line is one of the latest email trends and many organizations are already jumping on board. However, if you haven’t yet, there are a few things to consider before implementing.