ublue is entirely OCI images built on immutable Fedora, think of them as Fedora plus a recipe on top.

has anyone here used this who can comment on it?

I've been on uBlue since a couple of months. Initially, I just rebased to their silverblue-main image because it offered a more sane image to build upon as all of their images have already applied every relevant step everyone does to their 'Silverblue-systems' anyways; codecs, enabling hardware-acceleration, support for nvidia + secureboot when applicable etc. But recently I've started building my 'own' image using their toolkit and it has been a blast. I'm a huge fan of what NixOS and Guix do in the space of declarative distros. However, unfortunately, I had my reasons to not go down that route. The toolkit offered by uBlue enables me to have (pretty much) a declarative system on a more traditional -albeit 'immutable'- distro. If one desires reproducibility, atomic updates, very high security-standards and a pinch of declarativity to eliminate bitrot, configuration drift, unknown states etc; then one simply can't ignore uBlue's offerings as one of if not the best solution out there.

i see a lot of recommendations for nobara, but this seems to do a similar thing in a more convenient and reversible way

Nobara is great and does indeed have similar design goals; namely improving the stock experience. To put it bluntly; Nobara is to Fedora Workstation what uBlue (thus including Bazzite) is to Fedora Silverblue. To be clear; uBlue offers a fleet of different (base-)images; thus enabling everyone to use their favorite desktop environment on their 'Immutable' Desktop; even those beyond GNOME, KDE and Sway that Fedora itself supports on their 'Immutable Desktops'. So in that sense -perhaps paradoxically- Nobara is more rigid on install than uBlue, while the latter is the one referred to as 'immutable'. It's perhaps important to note that uBlue is not a distro; at least not in the traditional sense:

"This isn't a distribution, you can always rebase back to Fedora without reinstalling. This is a unique relationship between an upstream and downstream that is popular in cloud, but still new to the Linux desktop. "Custom images" seems to be a decent place to start since that's what people call them in cloud."