Or is it still the noob trap item?

Honestly I've never felt that mana hungry as Ezreal, his costs are really high. Unless Manamune becomes used primarily for being able to stack it for damage I doubt it.

Muramana is incredibly strong. Kind of like having a sheen proc on EVERY ATTACK. I think it's worth a try.

I'm a little afraid to try it on Corki. Still just feels like I'd be dumping too much into useless mana until Manamune is full. What am I going to use the mana for? Missle spam? Trinity Force grants enough mana for that. Muramana looks pretty sweet, but again, why would I build that much mana to begin with?

I actually run out of mana on Corki quite a bit, especially mid-game when pre-fight poking lasts a long time. Late-game it's not that much of an issue.

I ran the math and found that with a Triforce, Muramana gives +38 AD from its passive (for a total of 58, which is close to an unstacked Bloodthirster). With its toggle active and at full mana it's added an extra 114 damage, which is enormous, although this obviously decreases with use.

In terms of adding to that initial burst it actually looks much more cost-effective than a Bloodthirster.

you do realize that muramana can be toggled to have your auto attack and single target attacks to consume 3% of your current mana to do twice the amount drained into bonus magic damage.

so if you have a large mana pool, that means you'll be doing quite a bit of damage per attack and improve your single target abilities to do even more damage.

and it will drain your mana fairly quickly especially if you have additional attack speed.

put it on ryze and watch his rune prison and overload do much more damage. by having a higher mana to damage ratio

It just came out, I doubt we can really say for sure.

It is worth trying. Corki and Ez can't proc it on most of their abilities, but a single target champion can blow through a lot of mana fast. (does deal a lot of damage tho)
People who use it need to learn when to toggle it.

Corki has no single target spells (his R is an AoE)

It's likely better on Ez than corki, but it's best on AD Casters

Yorick (OMG E Burst)
Pantheon (The spears)
Urgot (The Q's)

It works on autoattacks too, according to the description (though I haven't tested it).