Manamune is an average item, but has a high Efficiency score due to its low cost (2400g).
If you're going for the Manamune build, buy pickaxe first: it will help with CSing and Poking.

I usually go Rageblade after Manamune for it makes easier to stack the Tear (more Attack Speed) and to trade. As soon as you upgrade your Manamune into Muramana it applies its passive on Muramana too.

At 3 items, we can start differentiating between different Manamune builds:

Muramana+Rageblade+BORK:
Not a fan of this build, but against tanks it could be good. It has good Burst Damage, good DPS but no Utility.

Muramana+Rageblade+Gunblade:
Build it if you need sustain for frequent fights. It has great Burst Damage, decent DPS and good Utility.

Muramana+Rageblade+DeathDance:
It has really low Burst Damage, DPS and Utility, but it makes you tankier and able to survive to some assassins.

With 4 items, from now on we will have these main builds:

BORK build:
Adding Nashor to the build really steps up Damage output and Utility. Go with this if you are against a lot of tanks.

Manamune AP build:
If you're going to carry and you need AP damage to balance your team's damage or to counter enemy's defences, go with Muramana+Rageblade+Gunblade+Nashor (and after Deathcap).

Manamune AD build:
If you're going to carry and you need AD damage to balance your team's damage or to counter enemy's defences, go with Muramana+Rageblade+DeathDance+Runaan (and after Infinity Edge).
If you don't need the survivability from Death's Dance (if you're not against assassins or burst champions), you can go with Muramana+Rageblade+Gunblade+Runaan (and after Infinity Edge).

Finally, at 5 items, we can have these builds:

BORK build:
It is still better with AP from Gunblade and Nashor, for it has more Burst Damage, DPS and Utility.
If you need survivability against Burst Damage go with Death's Dance.

Manamune AP build:
The Efficiency Score is lower, but it has amazing Burst Damage, DPS and Utility. Go with this if you need AP and you're carrying.

Manamune AD build:
Great Efficiency Score, amazing Burst Damage, DPS but no Utility at all (with Death's Dance). Go with this if you need AD and you're carrying (with a good support).
If you don't need the survivability from Death's Dance (if you're not against assassins or burst champions), you can go with Muramana+Rageblade+Gunblade+Runaan+InfinityEdge to get the most stats and some Utility as well.