The feature itself is great. It records the last two hours by default and lets you easily create clips from that. The editor is right there in the Steam overlay, it's pretty great.
I only used it under Linux, and that's where I'd say it is still very much a beta experience. I have an AMD Radeon 7800 XT. Most of the time, Steam picks up on its hardware acceleration - sometimes it doesn't. When it doesn't, it falls back to CPU encoding (obviously) which occupies around 3-4 cores on my 7950X3D to record 3440x1440 at the highest quality setting. GPU encodes are H.264 even though the GPU is perfectly capable of encoding AV1. Performance impact ranges from almost zero to as much as 30%, which seems a bit excessive. On some games that have a splash screen (Sea of Thieves for example), all it will record is said splash screen, even when it's not shown anymore: you get gameplay sounds, but the video is just a static image with mouse cursor artifacts. It didn't record sound from one of the microphones I tried. After swapping it out for a different one, my voice is being recorded. At least one session the shortcut for saving a clip just resulted in an error sound instead of a clip being saved.
So it's a bit disappointing so far. Yeah, Linux shenanigans and relatively small user base, but Valve out of all companies should treat Linux as a first-class platform. Yes, they do a lot for Linux, with Proton and whatnot. But ironically Steam itself is only in an "okay, it kind of works" state. No official packages for anything but apt-based distributions and Wayland (scaling) support is meh at best.
It did seem to work a lot better on the Steam Deck with very little performance impact in my short testing, so there's that.