Does anyone have an opinion on what would constitute "familiar"? Would you say this implies having been there, Having a visual image, or just knowing the name of a location?

I'd use the term Familiarity as it's described in the Teleport spell, so you would at least need a description.  Since you need to be familiar with the location on the plane (not just the plane itself) you couldn't just try to teleport to say, Elysium.  You need a target destination on the plane, which someone has either described to you or you've seen on a map at minimum. 

I'd say just knowing the name would not be enough, either.  So to get to Sigil, you would need to ask to teleport to the Outlands, the City of Sigil, which you know to be at the top of the great spire in the middle of the Outlands, or is a ringed city in the sky.  Just knowing that Sigil is a city in the Outlands wouldn't be enough to get you there (so the spell would fail).  On top of this, even if the Int check is successful, I apply the rules for Teleportation to determine how on target the spell is (I do this for Plane Shift as well).

Familiarity. "Permanent circle" means a permanent teleportation circle whose sigil sequence you know. "Associated object" means that you possess an object taken from the desired destination within the last six months, such as a book from a wizard's library, bed linen from a royal suite, or a chunk of marble from a lich's secret tomb.

"Very familiar" is a place you have been very often, a place you have carefully studied, or a place you can see when you cast the spell. "Seen casually" is someplace you have seen more than once but with which you aren't very familiar. "Viewed once" is a place you have seen once, possibly using magic. "Description" is a place whose location and appearance you know through someone else's description, perhaps from a map.

"False destination" is a place that doesn't exist. Perhaps you tried to scry an enemy's sanctum but instead viewed an illusion, or you are attempting to teleport to a familiar location that no longer exists.

Question is, on a failure, is everything transported to the same random destination? Or could your bard wind up in the hells, your fighter in Pandemonium, and your ranger in the plane of water, all due to the same failed check? And is the d100 roll made by each creature, or just the user?

Quote from ScarSent77 >>

Question is, on a failure, is everything transported to the same random destination? Or could your bard wind up in the hells, your fighter in Pandemonium, and your ranger in the plane of water, all due to the same failed check? And is the d100 roll made by each creature, or just the user?

 the item says you all go to the same place, but going to different places could be fun

Whats the price of this Amulet in po? Ruffly

The description as written states you cast the Plane Shift spell on a successful check--but does not specify either your targets or that you otherwise must cast it upon yourself or allies.  Doesn't this mean as written a character wearing it has unlimited uses of a melee spell attack to transport unwilling creatures within touch range?  Provided of course that you make the INT check to 'successfully cast planeshift' normally without the random mishap.  Except that mishap would work out even better for a BBEG facing a group. To make matters even worse casting a spell from a magic item has no components--which means this would not be subject to Counterspell either.  So apart from the double dice roll of both a spell attack and Charisma save there is no foil against this within the rules apart from a DM familiar with this items intent from past editions, or a common sense sentence errata.

If you make the int check, yes it could be used offensively, just as the plane shift spell. Only if the DM allows though. Because to use the spell offensively the creature has to make a charisma save to avoid being banished to the other plane. But if you fail, you and the enemy would be teleported away. So good luck getting out of that by yourself 😂😂

Imagine accidentally getting sent to Carceri.

Some planes like the plane of Earth, Fire, nine-hells, etc. Could put an adventurer in a dire or even fatal circumstance, even in just one round if they aren't careful. I think the usefulness of this item varies greatly depending on the DMs discretion.

When you cast spells from an item, you don't generally use components.  Isn't that part of the reason for the items in the first place?

WOW            \O/

In our campaign our DM gave us the Amulet of the Planes with about a 60% of dying every time we went to a random plane. so far even when we have used it as our main transportation the closest anyone has gotten to dying is falling until they hit maximum velocity and teleporting with feather fall and barely surviving. That is when we learned that maybe a heaven-like place isn't the best to teleport to.

Also banishment to plane of fire dusted a mind flayer.

dc 15 Intelligence check (+7-8)... a little guideance(1-4), have an artificer with a touch of genius (+4-5), and a peace cleric emboldening bond(1-4), with a warlocks talisman(1-4)... heh.

faultless travel  rolling minimum on those.. +14, and a 1 int check which doesnt botch... ohhh yeah. ;)

familiar means you need to have been there at least once. for mist walkers, however, I let them just have an item from the plane they want to travel to

I don't think you need the spell component to use this amulet

Correct! Magic items don't require components to cast spells unless they specifically say they do (DMG Chapter 7).

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you know the one thing that has been really bugging me about this how come the art only has 15 planes if there are 16?

The House of Liars is a layer of the Abyss, not part of Sigil, so there is nothing stopping the amulet being used to Plane Shift away.