really depends on how it works

It's an alright feature, but specific implementations are make or break.

It depends on how it's balanced and in some games it's terribly balanced resulting in either too easy difficulty or a save scum fest.

When it's well done it can be fun to play around with skills and gear you acquire near the end of the game thus having more time with late game content.

Neither, it's utterly pointless.
Because what's the point of starting out as the end-game demi-god you're supposed to progress/work your way towards over the course of the playthrough, right from the get-go?
"The journey is the destination" and all that.

1. Doesn't hurt to have it. Though I lean hard towards narrative purposes (e.g., Chrono series, Ys Origins, Nier series, RE2, Undertale) or multiple branching paths (e.g., Bioware games, Infamous, etc.).

Obligatory Design Doc:
- NG+: https://youtu.be/YK7AuOjX0WE
- Branching Paths: https://youtu.be/Gdt5zCdXoSc

2. If it's well-designed and you enjoy it, why not.

The only issue I have with NG+ is with artificial QOL debuffs you unlocked in your previous playthrough with no option to opt out. For example, the differences between FF4 3D and TMSFE. Your map completion % is carried over between playthroughs, which is an extremely helpful QOL improvement for completing Namingway's quest for the Treasure Hunter augment. However, starting a new Lunatic playthrough with TMSFE, your map completion is reset to 0% and it just becomes a hassle to redo everything again especially if I got 100% the first time. I was completely discouraged and haven't touched it since.

In FF4 3D:
* There's an artificial limitation of 3 playthroughs, which is annoying, especially if you assigned augments sub-optimally on your first playthrough (and there's no way to re-assign them).
* You get a reward for clearing a map 100%. If you get this reward on one playthrough, you can't get it on New Game +, and the rewards are consumables which (for the most part) don't carry over. (At least if you give Cecil the Bless augment, you can use it to eliminate the need for Ethers in the mid-game; add one Soma Drop on Tellah and you can spam Meteor in what would otherwise be the most annoying part of the game.)

Also, I am reminded of the SaGa 3 remake. New Game + gives you the option of keeping everything; saying "No" results in you not even keeping jukebox songs, and 2 of the songs will be forever unavailable on that file. (Also, New Game + is the only way to play Hard mode, which really should have been unlocked from the start, as that's closer to the difficulty I'd expect from a SaGa game, while still being easier than the harder entries in the series. New Game + without keeping everything is the only way to get a fresh start on the harder difficulty, which feels like how the game was meant to be played.)

Of course it's generally a bad thing. I mean, why would optional content that can offer expanded replayability to someone ever be considered a good thing? .... If you've already completed the journey previously than what does it matter? Is new game + affecting you in some way, or preventing you from starting from scratch when you want to make that journey again?

Whether you like new game + modes or not, there's very few instances where they're a bad thing. It's an optional mode / setting, and under almost all circumstances it has no affect on you starting a new game from scratch, or anything like that

Bad from where I'm standing. For one, it means the developers expect the game to be replayed, which is something I won't do, then it means they cater to those who play for the challenge, which again I don't, and then, and perhaps mainly, it typically means there are things (gear, skills, enemies, maybe even locations or whatever else) that are locked away during regular play and only become available for those who do replay and do so for the challenge of it, so in some ways the game won't quite be fully completed, or worse, experienced, if you don't do that.

As for whether you should dive back into a game with NG+ after not having played it for a while, depends how well you remember it and how hard NG+ is...

*nods head to all of the above*
Since this has become a thing in games I've never been tempted to do a NG+ run, especially right after just beating a game. At that point, I'm good and ready to move on to the next game.

I mean, god forbid a developer would want you to replay their game, or at least provide potential replay value for those who do like something they worked years on enough to replay it... And that goes beyond just new game + modes.

Nothing wrong with doing that. If you want to be a "one and done" gamer that's fine, but what you're saying is "I don't like replaying games, so I think this optional new game + feature is bad" when there's nothing objectively wrong with it in the vast majority of cases.

It's sad but these people are clear examples of what the anti-consumer studios want their audience to be like. Someone who'll eat up whatever shit they lay out in front of them, and then promptly move onto the next meal (game).

That's the typical modern gamer these days.

Depends entirely on the rest of the design. I appreciate replay value particularly if the design of combat/gameplay loop is fun. It can be a way to say "hey we don't have any more story content or budget to make levels but we know this is Tetris-level addictive....so would you like to play again with minor tweaks?" Not every game needs NG+ but the other ways to add length such as fetch-quests or collect-a-thons don't appeal to me.

As I said though it's a lot of factors and not every game needs one. If nothing is changed and it's just God Mode it gets boring without branching level/story paths/multiple endings. If drop rate increases or sequence breaking becomes an option that might be fun. I'm a fan of the idea of NG+ but there are so many factors to determine if it makes sense for a particular game. Finally, if it was a painful slog to get through your game one time and I didn't have fun, 37 secret optional bosses still won't make me play it again.

I'm not a fan, unless there is a difficulty raise specifically for it. Challenge to some degree is important to me so I can feel engaged in the gameplay. Even games like Mass Effect are super easy on their highest difficulty with an already leveled up character.

That said I don't think it's "bad" really, some seem to really enjoy blowing through the game a second time. I hope they enjoy it and get what they want from it.

wth is new game +?

I love NG+, there's more often than not, some late game equipment or skills you don't really get to use that much.
In a couple games you can swap the model and texture of weapons or armor pretty easily, but modifying skill requirements, at least for me, is too much. :P

Beginning the game as a walking apocalypse with the best equipments and an already maxed out build can be quite fun, plus in many RPGs you can set the level of you enemies to your own, so the game doesn't become that much of a cake walk.

JRPGs make you feel especially powerful, take Persona 3 (or any Shin Megami Tensei game honestly) for example, being able to summon Satan right from the get go to wipe your oppponents in a single hit just feels right. :P