It's good if you have to be tanky for some reason.

If the other team has a kass, kha'zix, etc, you need to be able to take a few punches and still hit back. So tanky mana items with a muramana let you spam your crazy good utility spells, absolutely MURDERDEATHKILL one person, generally good stuff. Is it the best way in all situations? No. But it has a place.

(It's also hilarious to 1-shot ezraels..... there are other squishies, but f*** you ezrael)

The only real advantage it has is during early/mid game. If you can catch people with your combo yeah, it will do a ton of damage and kill most squishy targets.

The problem is late game though. Late game you won't be auto attacking all too much in team fights so the bonus AD is pretty irrelevant. So the only advantage it has is bonus damage on your ult. However in team fights you will be using your Q, W, and E a lot more than you will use your ult (obviously). So getting 125 AP from Seraphs plus a very good shield is just so much better. Your ult will already kill carries late game regardless, so you may as well just build Seraphs and be able to do more damage to their entire team since Q, W and E are all AOE.

Overall, it's just pretty gimmicky and not really worth it over Seraphs.

Muramana is actually pretty core in a finished build for syndra right now, Whatayabrian is just behind the times.

How early would you build a tear. I mean, until you build into a maramune and then charge 750 to it, it really doesn't give you any damage.

First off it increases the damage of every single orb on her ult and since you can throw it with a total of 6 orbs, that hurts a lot.

EDIT:
[Secondly it also increases the damage on her Q as long as she only hits one target, so all in all I'd say it's just as situational as any other item you buy.]
Incorrect, read my post on next page.

It's fine, you just don't build the manamune until your tear is fully stacked. Honestly, you spend 700g for early mana and then continue to build standard AP items. When the tear is full, a 1400g deviance from your build gives you about 360~600 damage on your ult.

She spams her spells just fine, so there is no reason to build the manamune until the tear is full.