As a diamond 1 Assassin player, the only Assassin champs that I forsee using this item are 
and 
.
A lot of Fighter champions with heavy damage will make better use of this item than most Assassins will, champions like: 
.
You heard it here first, remember that! I also predict riot will end up changing this item in the near future.
MisterHealYoGirl3 votesIt could definitely push some of the more assassin-y fighters (Jarvan, Master Yi, Wukong) into full assassins, but I think it's pretty well-defined as an assassin item.
N00B PWN3R1 votesYes, Jarvan will be another good one.
MisterHealYoGirl2 votesBut I think Jarvan can ALREADY be built like an assassin (mid AD Jarvan is totally a thing). Most of the examples you listed above (Gangplank and Hecarim, especially) probably won't use this particularly well; Hecarim builds fairly few damage items and does almost entirely AoE damage in teamfights with his helicopter spam, W, and ult, and Gangplank is focused around getting AoE barrel crits, not on assassinating one target.
N00B PWN3R1 votesThe three champions I named and J4 will be capable of building this item as their first item and taking over the lane easily.
MisterHealYoGirl3 votesI really don't think so. Straight AD and flat armor pen just aren't particularly sexy stats against tanky laners, and against equally tanky lane opponents, it just doesn't offer that much. The passive is only good on people who can reliably go all in on their lane opponent or target and take them 100-20, and none of these champions can do that early. Three, four items against an ADC, sure, but it's not a very lane-dominant item.
N00B PWN3R1 votesYou will see soon enough.
YuGiHo2 votes
N00B PWN3R3 voteslol. no.
LaughingStapler1 votesThis sounds strong on Nocturne.
1374344 votesNocturne is an assassin however. He is just not a burst assassin.
Jileti1 votes[{quoted}](name=N00B PWN3R,realm=EUW,application-id=3ErqAdtq,discussion-id=W026Re6L,comment-id=,timestamp=2016-02-09T15:12:22.487+0000)
As a diamond 1 Assassin player, the only Assassin champs that I forsee using this item are
A lot of Fighter champions with heavy damage will make better use of this item than most Assassins will, champions like:
You heard it here first, remember that! I also predict riot will end up changing this item in the near future.
Erm. Zed?
N00B PWN3R4 votesErm. Zed?
This item is bad on Zed, much better options.
Definitely Bait2 votesAre you assuming this will be only a rush item? I could see zed building this fourth or third item in. The proc does pop slightly before Zed's ult. So you get the burst of 25% missing hp + 90 and then the normal ult burst (If they don't qss or zhonya's). Not to mention that small bit of movement speed can come in handy while escaping / split pushing. Although it's a minimal increase in speed with a lot of movement speed stacked (Swifties).
It should actually be good on any melee champion that can reliably burst targets to under 1/2 hp or 3/4 in 2s (Execute range for the item). So things like 
could abuse it.
N00B PWN3R1 votesThird or fourth? Still better options for Zed.
Definitely Bait2 votesTypical core build is
then you've got optionally Maw or Mercurial Scimitar or very rarely Hydra / Shiv when your pure split push.
What options do you believe beat out the extra burst from the proc and passive movespeed?
I'm real interested in more detail explanation of the exact reasons it won't be built and the alternatives.
N00B PWN3R1 votes
Definitely Bait1 votesIt's a bit old but https://www.youtube.com/watch?v=4nCZj3SXU80 still remains valid. Crit zed isn't exactly meta. It can be viable but it's rarely used. The nerfs to his animation cancels have most likely made a triforce build worse.
Not to mention the new item allows a all in without ult to force a zhonya's. After that you can just ult to finish them.
N00B PWN3R1 votesYes, 1 shotting enemy ADC in 0.5 sec is so not good, yeah, I don't know what I am talking about, I am only in the top best 5 Zed EUW on op.gg :^) you are right mate, go build Duskblade.
Definitely Bait1 votesYou'd have a lot more credibility if you weren't posting on a level 10 account that only does bot matches.
You've also yet to provide support behind why duskblade will fall short of building trinity force and infinity edge. Items which are only built 1.2% and 0.02% of the time at diamond elo and above (1.2% for inf edge, 0.02% for trinity force).
At least provide some supporting reasoning behind your statements. Especially when they're contray to data provided by easily viewable match historys and statistics.
N00B PWN3R1 votesLook up this account after 3 months for your credibility.
Definitely Bait2 votesEven if you are credible choosing infinity edge and trinity force is still contrary to what every other top 5 zed in every region commonly builds.
That's equivalent to saying the new assassin item will be about as useful as building 
at the low end and 
if you take a middle ground between the two.
N00B PWN3R1 votesI don't care what other Zeds build, I have my own playstyle, they have theirs. There are multiple ways to greatness. I won't be building Duskblade on Zed, I don't see how it adds anything of value. Zed has delayed damage with his ult, I'd rather have more upfront burst damage. Duskblade will be great for Talon and Kha'Zix, but that is all when it comes to Assassins. And I don't get this fetish of rito trying to make Assassins able to kill tanks my job is to 1 shot their squishies, not Tanks. Regardless, this item is not even 50% as effective vs tanks as old Last Whisper was, so this item is an epic fail on all fronts, from what rito said they had in mind (helping Assassins vs tanks).
Ðelofasht1 votesI agree with your assessment, but I actually said that a couple weeks ago when it was first spoiled and there was another thread about this. Especially fighters who need AD and a bit of ArPen as well as movement speed, such as Illaoi will find a good benefit from this item. They also tend to only get one or two autos in a fight on an important target, relying more on their abilities.
SailorPsycho1 votesAs a diamond 1 Assassin player
Love how you post this from a smurf that has some really terrible bot matches...
N00B PWN3R1 votesStay jelly, I am used to jelly haters.
SailorPsycho1 votesStay lying kid :)
Daxanater1 votesI believe this is correct, fighters will use this way more. I really want to know the exact reason to why they are making this item, I heard that
Apparently there are some assassins too over powered and underpowered, which is like the most bs reason.
A lot of the items don't really support assassins. This I can agree with but why this item? this just gives squishys a harder time.
The Bearded Bard1 votesI think it'll be good on Riven.
Also not seeing it being good on GP considering he's gonna waste the passive a lot on his Q.
N00B PWN3R1 votesHis Q won't proc Duskblade.
The Bearded Bard1 votesAh, my b. Thought it was counted as a basic attack. Still think it'll be good on Riven.
N00B PWN3R1 votesDepends how fast you can animation cancel tbh.
Salron1 votesGP needs to get to hit his midgame powerspike
He can't afford to delay that just for Duskblade
Gigahappytown1 votesit activates on first auto attack deals missing health damage and has a 1 minute cooldown
ITs only viable on assassins as 90% of the time when they auto attack champions they going all in and they have the burst to make it worthwhile
its VERY rare for this item to deal above 200 and above damage its super countered by armor and LW isn't viable on assassins anymore.
Ven0mousduck1 votesNoob question. Does Master Yi's Q proc it, second how many people can be proced? Team fight q will it proc on all 5 or even 2-3 of them?