I wouldn't say that mana is overrated and I agree on what Kaarin said.
In my opinion you could rather say that other "resources" such as actions and spellbookpoints are often underrated especially by newer players.
More creature actions give you more opportunities to react to what the opponent does e.g. by guarding, applying conditions on enemy creatures, etc.
Familiar- and spawnpointactions let you plan and cast more spells which is always good for either reacting or putting pressure on your opponent eg. by getting an early creature advantage.
If you use your spellbookpoints efficently you can counter more of your enemies possible moves which also can save you mana if you have a very mana efficient counter in your deck. In addition to that an efficient spellbookpoint planning could be the small advantage in longer games you have over your opponent to win the match.
You can have as much mana as you want, if you don't have the actions to use it or just not the right spells to spend it on it gets almost useless. Also having good ways of reacting to different situations often let you getting advantage even if you spent less mana than your opponent.
In the end I think a spellbook should find a good balance between all three resources (mana, actions and spellbookpoints), with corresponding modifications depending on which mage you play and which strategy you pursue, in order to be competitive.