Swordmages are amazing. I'm not going to say they're better or worse than any other defender or any other class because I abhor such conversations, but Blackfang108 is right: If you're playing a Swordmage the same way you play a Fighter or Warden, you're doing it wrong.

My swordmage (shielding of course) will mark an enemy, use one of many abilities to teleport to the other side of the room (and for the record, she's human, not eladrin, all her teleport abilities come with the class even without aegis of assault), pretty much force the target to attack someone else and have its damage negated, the triggering of which will give the swordmage some decent bonuses due to some feats.

If someone manages to hit her, she retaliates with Frost Backlash for 3[W]+Int frost damage. If someone manages to hit a nearby ally, she can use her Corona Guard paragon encounter to basically make the attack whiff. If someone gets her with a ranged attack, she can teleport right next to said ranged character. If she gets disarmed or captured, she can call her sword to her from a mile away.

So yea, in my experience, swordmages are awesome.