Luckily shadow mage is able to go many routes depending on how you wish to play. Currently I play support aka healer and it's quite effective. Seryss, another sm, plays as a dps type SM and she's really strong.
Depends on how you like to play SM's can go either way and be strong just pay attention to your build and plan your skills before placing them or you may have to re-skill or re-stat later on.
Edit - for PvP, they're both the bigger threats. As they have many types of heals and DoTs to be a problem for any other class.