(woah I dunno why I made a wall of text but here it is)
Yeah i'm rocking muOS on my 35xxSP (I hate that name everytime I have to type it), and its pretty dang great. In not much of one for the curated library experience with like lush game covers and cool flashy layouts so I dont have much insight to offer there. I downloaded a Star Trek LCARS theme and that was good enough for me.
MUOS has had no problem reading roms off the main boot SD card (which thanks to their rom directory rules is super grab and go, select a rom, select a core your first time and you're off) is really seamless. You can set favourites etc its very intuitive with on screen button guides. By just launching retroarch on its own you can load roms off the second SD card even if its not formatted to their ROM directory rules which in my case is good because me second SD is just my mass storage card and isn't organized, so its a bit more of a manual Retroarch process to play but not problem for me.
The SP hardware is fully supported, there is lid close detection and you can tell it how you want that handled, I have it set to save my game state and power down after a set time. My play sessions are typically 3-5 minutes long when I have a brief moment to myself (job and family = gaming atrophy) so its nice to just close it up when I am needed and not have to worry about powering down etc, then when I power it back up again it does take a minute or two to boot but then instantly brings me right back to the game and state I was in when I left. Very nice and it saves me having to constantly charge a dead device I forgot to power down.
There are some omissions like bluetooth audio, but I've never really had a good experience with bt audio and emulation on handhelds in general so its not glaring for me. I make do with wired headphones if I need them, but the audio response is very quiet. Not sure thats MuOS fault. I think there is also support for BT controllers, but I've never tried that. Nor have I experimented with HDMI out or the like.
The team making muOS are pretty cool and seem like they have a good vibe. Recently they announced they were actively making changes to the firmware to block a third party application that automagically downloads roms, and it caused some discord drama. I get their motivations, and their desire to protect their work from unwanted ninty litigation, it just kinda seemed like an over correction to me. Just one parasites opinion though. I don't think they are being dicks, just looking out for their hard work and I respect that.
On the side of general purpose computing theres a number of convenience apps built in, an MP3 player, a video player, some file management tools, a terminal. Theres also an integrated backup and restore tool if you want to nuke and pave. There's also a web based file server if you want to transfer files over wifi which has been SUPER handy.
On the whole I feel muOS has been a really good experience on the ANBERNIC RG35XXSP and if you're on the fence about trying it consider this a recommendation.
Thats a backrooms I'd almost enjoy being cursed to be lost in.