I have all my 3 elemental spells at 4, finally. Pretty pretty powerful. And I dont find it to be so hard on the mana - but of course, Im pretty conservative with its use, trying to save as much as possible its just sometimes unavoidable, like when Griss falls or a gang go after me.
But it's pretty manageable. Of course, having 400+ mana helps a great deal.
But here is the thing, if you're not ready to play without zaping everything that moves, you cant play Mage. You cant just expect to throw a fireball at everything that crosses your path.
By my experience with it, Im sure there would be many people that will struggle with this, which is probably why so many people complain that its hard or that is "weak in the early game".
Im even thinking about removing the points I placed on the extra mana skill. I'll do it once I reset