I bought this on sale and am not too impressed by it in its default use cases.
Because Lossless Scaling's frame generation requires quite a bit of GPU horsepower (thus dropping your native FPS, which it uses to generate more frames, by anywhere from 10-25+), it seems the only real use case is for games/systems where you are severely CPU-limited or just old games with FPS caps where your GPU has tons of overhead to generate the new frames without losing native frames.
Like in Kingdom Come: Deliverance, I go from 90+ FPS native to 70 FPS with LSFG on. The FG can bump me up to 140 FPS, but it feels worse than native 90 FPS because of the input lag and reduced responsiveness of the real 70 FPS (with FG increasing it to 140) vs the real 90 FPS.
However, there is an interesting option where you can run a secondary GPU to generate frames through LS while native frames are rendered by your primary GPU. So you can get frame generation without any/minimal hit to your native framerate, which significantly reduces input lag with FG on. Best of all, the secondary GPU can be a fairly cheap card.
I still have a 3080 and am strongly tempted to try this, as I don't have MFG. At the very least it seems like a pretty decent stopgap if you need more frames on modern games but don't want to shell out for modern GPU prices.
