There is plenty - and varied - information available on using symbols in subject lines. Below Listrak Account Director Karen DiClemente shares some thoughts and best practices:

When using symbols in subject lines, retailers are primarily trying to drive higher open rates. I encourage ALL clients to A/B test subject lines with and without symbols to see if the symbol actually made a significant impact on open rates.

Prior to using a symbol, here are a few questions I tell clients to ask themselves:
How does using symbols in a subject line impact your brand (will they be seen as juvenile? Is this the brand impression you want your subscribers to have?)
Does the content of the email warrant a symbol in the subject line?
Make sure you are using symbols for a reason (not just to “use a symbol”):
Symbols can be used as a separator or punctuation
Symbols can be used to replace an actual word (ie. using a ♥ to replace the word “heart”)
Symbols can be used as a decorative design element
Not all symbols render in all email clients – make sure you are using a rendering tool (ie. Email on Acid or Litmus) to ensure the symbol you chose will display correctly

The guide below provides some more detail:

Subject Line Symbols by Email Client

Outlook 2010 – all symbols render correctly; color does not change

Outlook.com – all symbols render correctly; color changes based upon specific symbol

Comcast – all symbols render correctly; color does not change

AOL/AIM – changes the 8th symbol; color does not change

iOS (iPad/iPhone) - all symbols render correctly; color changes based upon specific symbol

Windows Phone - all symbols render correctly; color changes based upon specific symbol