It is broken, I have been playing ryze alot on the pbe because of this. I have also created a thread on this as well. Basically my idea was to increase the mana to damage ration to 3% from 2%, and reduce the active to 3% from 4%. This may achieve some balance, and still keep the cool flavor of this item.
I have had this item stacked between the 25-30 min mark in every game so far, but I also find clarity to work well with this item. after a combo you can get your mana back to 100% to keep the high burst. In that sense its better than a ignite late game for damage. Its quite effective and keeps you in lane long enough to charge this thing quickly.