If this was a 10$ cube I'd say "It's awesome for a cheap cube". At 4x that price it is not something I would advise anyone to buy.
My first reaction was that this was a Tornado knock-off. The problem is that it is not and does not aim to be inexpensive. The pieces are hard and blocky and feel like a cheap cube. However, it turns great, and is both faster and more stable than a cheap cube tends to be (Cyclone Boys, Solar S, I'm looking at you). But at twice the price of most flagship cubes it doesn't bring the quality and feel of a Gan cube.
Nor does it have the performance of a flagship cube. The corner cutting is in line with that of most cubes coming out since 2019, but it does tend to hard-lock when it doesn't cut, which happens during normal solves (at least if you're as bad as me at turning).
The transparent inner surfaces look awesome but make weird squeaks for some reason. Swapping them does change the feel of the cube a bit, but frankly not more than adding denser vs more liquid lube. It doesn't solve the issue with lockups, nor the overall blockiness of the pieces. Magnets cannot be adjusted and tend to be on the strong side. That doesn't always work to its advantage, and makes slices less smooth than is comfortable for my Roux solving.
A note on customization : the DIY approach is an interesting idea, but it means spending 1+ hours taking off pieces trying to not destroy the plastic inserts to make them usable once again. The springs get entangled and require 20 minutes to separate out and insert, and having to replace the "screw" stopper entirely to change spring compression takes a ridiculous amount of time. It reminds me of the core-swapping, spring changing and Frankubestein hybrids that cubers used to put together 6-7 years ago. Today you can just buy another cube if you want a different feeling. And given the price of the S3R, you can buy 2-3 world-class cubes for that price and need a lot less time to set them up. Fun fact: the center caps are very well designed, and come off very easily compared to cubes where taking off the caps quickly would be more useful (as you can change their settings quickly). Interesting that of all the places where they could invest good design, this is where they decided to do it.