I was trying out FSR4 on my RX 6800 XT, Fedora 42. Works really well and it easily beats FSR3 in visuals even on Performance. It does have a significant performance hit vs FSR3 though but it still works out to be a bit faster than a native rendering on Quality.
It is (if we talk about FSR as upscaler tech). But it wont help in CPU bound scenarios where the GPU already has to wait for CPU.