Martin Meany on Moz.com did a subject line test comparing commercial, functional, and emotional subject lines. The commercial version focused on appealing to the financial aspects of the purchase decision. The functional version stuck to the facts, and just described what you would find when you clicked-through. And the emotional took a “softer and fluffier” approach.
Interestingly, in the test, he found the biggest improvement vs. the control when he used the functional copy, with very little improvement from the commercial copy and a negative impact from the emotional messaging.
Martin’s takeaway was that, more than anything, you need to make sure you test everything!