Total of 18 heros,
10 Creep
8 UD
6 of those are tanks (Rue, Nephillium, Ascendant, Fenris, Treant, Bane).
4 of those are invis-seeing spell heroes--and tanks (Fenris, Rue?, Nephillium, Treant) double pwnage there

3 of those can almost one cast her (Ardin, Shrillwing, Icespinner).
Thats 9 that can take her and...
9 she has a shot at (Gravel, Incarnation, Defiler, High Oracle, Shaman, AM, Scarab?, Crusader, Emberwraith).
9/18
=50%
So as you can see she isn't op! not even close. Also, even if you did have amulet or something for 1 of the 9, it woudln't do much good becuase none of them could really touch her anyway, except maybe Scarab.
point of contention: Inf is not countered by amulets or other invis seeing items, she countered by high armor/hp heroes or heroes with invis-seeing spells AND high hp (i.e. tanks and a few DPS).
However, while im on the subject of items there are two that either need to be removed or nerfed badly. This items are the scroll of neutralization and the wand of negatation. Earlier I said that amulet doen't completly ruin Inf, It doesn't, it just makes her visible like every other hero, and its 500g not 160g. Its one thing if it makes you visible, its another to completly wipe a hero with a 160g item. Could you imagine Alexander with a dozen of those things? he'd be impossible to kill, also think of what it does to dot heros like Defiler or IS (which we all know and love). It completly dismantles them. I was playing a game few days ago and had some of those wands with SB. IS and treant were against me...and they couldn't touch me!...if IS put BF or IL on me I simply removed it for a retarded 160g. Actually its more like 50g (3). Something needs to be done, this isnt right, its a gay and cheap way to win (against heros anyway).