Why many people are switching to NixOS ?

I used NixOS for a couple of years. My experience is like this:

It is a rolling release (mostly)
You write a declarative configuration for your system, e.g., my config will say I want Neovim with certain plugins, and I can also include my Neovim configuration
It is stable, and when it breaks it is easy to go back
Packages are mostly bleeding edge

Note that there's both the rolling unstable channel and a bi-annual stable release channel.
Important to note that NixOS has both a rolling release and point release version.

I didn't get it either, but this video does a pretty good job explaining why it's different: https://www.youtube.com/watch?v=DMQWirkx5EY

I tried it about a year ago and I don't know it did not convince me. Yeah it might be great for some niche developer oriented needs or deployment but for a normal OS usage, meh. I kind of see it as a current hype, just like crypto/NFT before, and AI now. For normal everyday usage I find openSUSE Tumblweed much more suitable and much more widely applicable.

Here's the straightforward version of why I use it:

The entire state of your operating system is defined in a config file, and changes are made by changing the config file. This makes it super easy to reproduce your exact system many times and to know where all the many different configuration elements that describe your system are located.
Updates are applied atomically, so you don't have to worry about interrupting the update process and if it fails, the previous state of your system is still bootable. By default every time you change something, you get another option in the boot menu to roll back to.
Making container-like sub systems is super easy when you're familiar with nix, so you can have as many different enclaves as you like for different software versions, development environments, desktop setups, whatever without taking a performance hit. Old versions of stuff are very accessible without breaking your new stuff.
The package manager has a lot of software and accessing nonfree stuff is straightforward. Guix looks rad, but nix ended up being the more practical compromise for my usecase. I didn't want to have to package a heap of software the moment I made the switch.

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All I year about from the linux community is NixOS and btrfs, neither of which I have any interest in. It almost feels like someone with an agenda is promoting these two with how prevelant they are.

I like using btrfs with Arch because of the snapshots. If an update breaks something I can just boot into a snapshot from grub keep using my PC and solve the problem later. It's very useful... yes... very... you should try it... come... try btrfs... it's warm and cozy... INSTALL IT!
- I have tried btrfs in the past and when it goes wrong you are utterly shafted. You can't even mount it as a read only file system, it will just lock you out entirely. And the support isn't great, I ended up finding something that had a disclaimer along the lines of "only run this if you really know what you're doing", but obviously I didn't as the documentation didn't tell me enough to know. So the only people who could possibly know are the developers of the file system themselves. Anyway, I was 2 days in to trying to recover my data by this point so I gave it a go, nothing to lose - it refused to do anything. Great.
  
  So in summary I'm not going to try it again.

Glancing over the website, I thought it's an immutable OS, like Fedora Silverblue. I could imagine that it might be cool to use with Ansible and stuff. But for an average user? I can't really see the advantages in respect to the work you have to put in.

It is an immutable distro, altough it isn't image-based like Fedora's rpm-ostree.

NixOS basically replaces Ansible because the Nix package manager achieves the same goals already (configuration, deployment, ...).

But I agree, the work necessary to put into this non-standard distro makes it hard to recommend for a casual user.
NixOS is not immutable in the way Fodora Silverblue is, and way more declarative and reproducible than Ansible. But yeah it is not something you "need". Other distros work too, but NixOS is way more fun.

Because it’s the latest Cool Nerd Thing™ like Arch before it, and Gentoo before that. Most of the people raving about it probably don’t have much use for its features.

The features themselves are very useful for basically any user. Whether they are worth the non-standardness and issues that come with it is another question.
Solution without a problem. A cool solution but yeah.

What about Nix's financial issues? Have they been resolved yet?

To get it out of the way first: There are no financial issues. There are more than enough funds to continue operations as they are for a sufficiently long time.

What is actually happening is that a long time sponsor has indicated that they (understandably) no longer want to foot the huge bill of hosting the entire archive of binary caches ($9000/mo). Finding a more sustainable setup is what the community is currently concerned with.
There is no risk of operations shutting down any time soon, the NixOS foundation has funds set aside to continue even this unsustainable setup for at least a year. We just want to be more efficient with our and others resources going forwards.

That's what all this you might have heard of is about.

Btw, even if the binary cache were to go poof, we don't technically need it. NixOS is a source-based distro like Gentoo and source hosting is not a concern. The binary cache is immensely helpful though which is why we'd obviously prefer to keep it.
- I think AWS Gave them 12 months of free credit to host cache

It's insanely stable but you have to have a lot of linux/programming knowledge to do even the simplest things like installing/updating your software or making little tweaks. I played with it for hours the other day and I'm just too dumb to figure it out lol I think it's just a super stable highly customizable distro for power users and a lot of people like that. If you can get over the learning curve it's a pretty powerful and unique os

It's kind of funny because I'd put NixOS on a complete newbies computer for sure, and recommend it to an expert... But I'm less sure if I'd tell a random mid-intermediate Linux user to switch.

Like if Grandma wants Linux on their computer to do some internet browsing for some reason... I'd absolutely put NixOS on it because it's easy to manage the system for them... But somebody who is a little familiar with Linux already might be more confused about the differences. It's kind of the ultimate beginner distro and the ultimate power-user distro, but a bit awkward between those extremes, haha.
you have to have a lot of linux/programming knowledge to do even the simplest things like installing/updating your software

So, pretty much like any other distro
- Most mainstream distro's can do all of that without a CLI.

everyone

Now that's what I'd call a stretch...

Indeed, why would I switch, already have been running NixOS for 10+ years.
I'll edit. That was clearly a stretch

I keep seeing trends with Linux distribution like teenager looking for new fashion.

I think it’s mostly the very young Linux user who hope from one distribution to the another over and over whereas many just stick with what they got : Ubuntu, Debian, mint, maybe fedora.

NixOS is certainly interesting tho.

Atleast NixOS isnt one of the countless Arch based distros emerged since pandemic

I don’t know NixOS. My Linux machine runs Pop_OS and Manjaro.

What are the pros and cons of NixOS ?

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