Manamune gives 1 attack damage per 50 mana. With its flat 25 bonus attack damage, it provides as much attack damage as a B. F. Sword at 1250 mana. Taking into account how much Manamune costs, it is less efficient than a B. F. Sword until 1878 mana, although it does help with early game mana issues.
Multiple Manamunes will not stack any of its effects, as they are unique. While Archangel's Staff is effective with Manamune for stats, its mana bonus does not stack with Manamune's, nor does that of Tear of the Goddess.
Unique Passive: 2% of your maximum mana is converted to attack damage.
Unique Passive: Whenever your champion attacks, they gain 1 maximum mana (2 second cooldown). Whenever your champion uses an ability, they gain 4 maximum mana (3 second cooldown). Bonus mana caps at 1000.