I tried it a few minutes ago, and the experience is... unpleasant to say the least.
It has a interesting artistic style, retro style. And it's very cool because it brings new music tracks adapted to this new retro style. It makes many references to Super Mario 64, for example all the music in the game uses the SoundFont of the Nintendo 64, specifically the SoundFont of Super Mario 64.
On the other hand, the gameplay changes a lot, and in certain aspects for the worse.
We start by having a polygonal three-dimensional perspective of the game. This greatly refreshes the experience and allows you to play Celeste again as if it were your first time. Now, two problems. The camera has strictly manual movement, with some zoom-in and out.
This brings in it the problem that now you not only have to control a character that is difficult to control by default, as is the case in the original game, but you also have to move the camera. This even happens or makes you understand that it happens when you have to make precise jumps. This forces you to have to do so many things at once that you end up doing nothing.
As if that were not enough, the main problem this game has is the controls and the detection of solid hitboxes. The game has a big problem; In the original Celeste the experience was very smooth, you usually always have control of the game, and there are only a few times where you may think that the game has been the one that has failed you. The mechanics are very well maintained and this means that if you fail in Celeste, you will be angry with yourself for not being up to par, but not with the game.
Now in Celeste 64 things are very different, here you can notice that the game sometimes does not register your keys, especially when you are near an edge. There are moments when you can notice that you have pressed a key and the character doesn't care. This happens for everything, even when you try to take directions in the air. It seems like that the character cannot turn most of the time in the air.
Take, for example, a top view. For me to make a jump with a direction of, say, 45º, I would have to move the character to the right and up, then jump. That's a mistake that old games made and it made them quite unbearable. This gives you the feeling that the character is super stiff. Many jumps in the game are like this, or have strange/hard angles and shapes, and the fact that your character is unable to move in the air (sometimes yes, sometimes not), makes it almost impossible to enjoy.
On the other hand, we have the problem of hitboxes. Precise jumps in Celeste are often and require many attempts. Here a little jump can cost you 4 lives due to the fact that your character seems to touch the hitboxes with his face and falls. I could swear I've seen moments where the character, approaching a corner, brushes against the hitbox, starts surfing around it, and falls. Or times where the character would land on a box just to keep walking on it's own (no input given) and fall from the edge.
The same thing happens with the Z button, which is responsible for climbing. Sometimes, depending on where your camera is looking at, the character will NOT grab the wall. I think that it should do it regardless of the direction of the camera, but I am not a developer of video game mechanics, so I will not comment on anything regarding this.
So, together you have a super stiff character, who hardly moves in the air, a very complicated camera to move, quite painful hitboxes, and the game barely explains how to progress the game correctly (there is no tutorial, the game mechanics, including the hidden ones, you should already know them from the original Celeste) takes you out of the experience.
In short, with Celeste, no matter how difficult it was, I ended up having a good time because I always thought it was a question of needing to improve because in reality only a few places in the game gave me problems to hold on to walls or doing specific jumps.
However, with Celeste 64, it seems like half the difficulty of the game is retrying things until the key you pressed registers, or until your character decides to move in the air, and that's not a thing that gives you a good experience.
I understand that it is a spinoff game, and that the mechanics that cost so much to polish in the original Celeste are not useful here, and that they have not had time to polish them better. The game is beautiful, it's charismatic, it's free... But personally it's not fun to play, at least if you plan to make the hard parts of the game, which is what Celeste's challenge is about in my opinion.
TLDR; Game is visually appealing and music is a very nice touch, but gameplay is somewhat crude and jumps/moving/camera doesn't feel well polished.