I personally think Archangels staff is TOO expensive for what it gives you...

3100 gold for an initial 250 mana 80 ability power and only 50% mana regen. It takes long to get seraphs and since you have to use continuous abilities to try to get the item to evolve and with the low mana regen its pretty crappy. Then when it finally evolves the shield it gives tends to be pretty weak.