I loaded up my level 36 mage at the start of FoV and replaced the Archmage belt. She did have 100 Intelligence, and you probably only want 60-70 by that point (because of the Dragon Wizard set), so my stats were a bit high. She had 0.9% Mana regenerated per second, and a base pool of 850 - which is really low, because I hadn't been putting in any points into Spirit.
Even without the belt, I was in no danger of running dry on mana. I was using my skills as fast as possible, and while I wasn't topping off, I was still gaining more than I was losing.
Therefore, the belt is not a game-breaker by giving you unlimited mana, you can already effectively have unlimited mana without the belt.
Only Rangers have a noticeable benefit from fast regen, really. Warriors skills use little mana - and a handful of points into Spirit won't affect their ability to fight. Mages usually have so much it doesn't matter, but Rangers have fast-cooldown skills that will drain their mana faster than other classes.
