Originally Posted by Kersei
I don't think manamune belongs on Ashe, Ezreal, or Miss Fortune. Mana regen glyphs/seals + 21 util (if really needed) cover mana problems for them, so manamune is just an overpriced BF sword that doesn't build into anything.
But...doing the math, if you have one or more item in your build that has + mana, it becomes more cost-efficient than BF swords.
For example, Nasus. A standard build might include a Sheen and a Banshee's veil already. I don't have the stats in front of me but I think that is 750 mana total. Add in the + mana from manamune's passive, and unique, and you would end up with + ~2000 mana. Add in the character's inherent mana and you have a beastly 90+additional damage from one item.
For Ez, you build a sheen, or you're bad. Add in Manamune and that is still a bonus 80 damage from a 1900 cost item...plus you can put your blue runes into CDR or MR or something else useful!