It's literally a steam deck without the screen and a slightly higher build quality. If you've used a steam deck, you know what the steam controller is already.

If you DON'T have or have never used a steam deck:
firstly, my condolences,
secondly: it is the literal solution to PC couch gaming when paired with Steam Input.
The joysticks are very accurate and glide like butter. Their unusual position is very ergonomically friendly too.
The back buttons are extremely useful, especially when you can bind them to keyboard and mouse inputs and not just controller inputs.
The trackpads are the crown jewels and are extremely versatile.
Gyro aim is such a powerful creature comfort, along with the ability to have it toggle on and off when you stop touching the grip or top of the joystick.

I really wish Valve had the capacity to mass produce these and sell them in brick and mortar stores because they are everything ive ever wanted in a gamepad.

Most reviews on youtube are already telling the full story. My review is this: it's a good controller, comfortable to hold, with tmr joysticks, gyro, trackpads and back buttons. I use it to play controller focused games, and as a mouse/keyboard solution for htpc purposes. It does the job very well, can't complain. It's also "just a controller" with more features, so there isn't much to talk about in the first place.

It is quite possibly the least ergonomic controller I’ve used. I like to sit at my desk with my controller on my lap right in front of my stomach. The grips are so vertical that my wrists are almost at a 90 degree angle. The only comfortable way for me to hold it is at the top basically squeezing it, and at the bottom almost not touching it at all to get a decent angle on it. Even then I have to sit back to give enough room to stretch out my arms so I don’t destroy my wrists.

The joysticks are also very far inwards. My hands aren’t gigantic, but they’re definitely not small. I’m a 6’1” guy for reference. I’ve found that to comfortably use the joysticks my hands need to be positioned more on top of the controller than along the side. So pair the weird angle with my hands being on top of the controller and I HATE the ergonomics of the thing.

The trackpads are cool to some people, but I don’t care since my mouse is right next to me. The software doesn’t even work right and I’m still waiting for them to fix my issue.

But besides the horrendous ergonomics I like it enough to use it. Eventually when the elite series 3 comes out I’ll probably switch to it (as long as it’s got TMR)

I got one in the first wave.

The control sticks are solid. Very accurate and without any drift. I prefer the symmetrical layout over the xbox controller's design. It may be a bit large for people with tiny hands though.

The rear buttons are nice, this is my first controller which has them. They sit right under your ring and middle finger's first knuckle so activating them is as easy as just squeezing the controller. They have a high enough actuation force that you won't accidentality press them.

The haptics is the first thing that I noticed. The controller's sounds (start up and shut down) are made using the haptics, not a speaker. When you're using the track pad the haptics create a... texture. It's kind of like you're running your finger over a slightly raised grid, I'm not sure that it improves anything but it's a nice feel. You can also press down on the trackpad like the thumbsticks and it 'clicks', except the trackpad doesn't actually move and the click is simulated with the haptic motors. It fooled me for a few days until a friend told me what it was doing.

The gyro is accurate. I don't play too many games that use it, nor use the exotic control schemes that do... but it was fun to use in Subnautica 2.

The plastic texture is nice, I'd describe it as aggressively matte. It adds a bit more friction to the surface so if you get sweaty hands you don't have to worry about it sliding in your grip.

The battery life is great. I've used it, casually, multiple days in a row without charging and when you do need to charge the puck jumps up and magnets into place perfectly as soon as you waive the controller in it's vicinity.

The box it came in is also really nice. It's made of pretty thick cardboard and has a form-fit recess for the controller. I was planning on 3d printing a stand to hold it but the box works perfectly.

It's nothing life changing, but it's a solidly built and designed product and well worth the cost.