Shapeshifter is utterly useless without mods. It's slow, the forms are weak, and you lose access to all your spells when shifted.
Spirit Healer is pretty good, but you probably won't use all of the abilities. It's certainly worth having at least one in the group, but you can also use Wynne or Morrigan for this.
Blood Mage is great. You can get some very powerful abilities, and you can use HP in stead of Mana which is very useful if you use it right.
Arcane Warrior is grossly overpowered. With the ability to wear massive armour, a few defensive buffs, plus the entire AW ability line, you're virtually unkillable. And if you turn the AW buffs off you can cast (almost) as well as a regular mage.
One great (and obscenely overpowered) character build is to go Blood Mage + Arcane Warrior. First cast spells like a normal mage until you run out of mana, then toggle on your buffs and run into melee where you'll be untouchable. If you still need to cast spells at this point, simply activate Blood Magic you use your HP in stead of mana. This way you can take on almost any encounter with great ease.