I got a Miyoo Mini Plus last week from AliExpress and have been having a blast.
First of all I am very pleasantly surprised by the overall build quality. The plastics don't feel cheap at all and my main concern with the buttons ended up being unfounded. They feel great and i'd go as far as saying that the shoulder buttons feel better than my Analogue Pocket's. The d-pad is also surprisingly serviceable - how a $70 handheld can do what Xbox still can't after two decades, I will never know.
The IPS screen generally looks nice. Don't go in expecting top tier OLED quality, but it's bright and colorful and when you see retro games running on it pixel perfectly, it's hard not to be a little impressed. The only negative point was the horribly muffled built-in speaker but I found a cheap mod on Etsy which swaps it out for a better one. As a side note, the packaging it came in was slick and they are kind enough to throw in an SD card and cables, although I personally threw mine away to use a 128gb Samsung SD.
At first I was disappointed with the performance of the device. Lots of frame drops, screen tearing, ugly filters on the games. I heard that the aftermarket Onion OS was practically a mandatory install though, so that was my first port of call. I'm happy to report that it resolves basically every issue. Once i'd done a bit of tinkering with each Retroarch core (only a bit as OnionOS does most of it for you), I now have great emulation of NES, SNES, Game Boy Color, GBA, Genesis + Sega CD, NeoGeo + CD and PlayStation 1. It's capable of other pre-PS1 consoles too but those are personally what interest me. There are some very minor performance dips on PS1 but a lot of that is present on a real PS1 too. Mild overclocking via Retroarch seems to resolve most of it so I don't think it's a case of the device being underpowered.
The game changing feature though (and in a lot of ways I mean that literally) is OnionOS's Game Switcher. This functions like Quick Resume on Xbox Series X. With the press of the home button on the Mini, suddenly I can scroll through snapshots of exactly where I left off in a dozen games. Right now I have: Pepsiman, Gunstar Heroes, Metal Slug, Crash Bandicoot 2, Tetris Blast, Super Mario Bros 3, Wario Land 4, Silent Hill, F-Zero, Chrono Trigger and Advance Wars, all running and ready to dive back in. The feature is fast and to put it into perspective you can swap between games in less than 5 seconds.
The question people may ask is: why did I buy this device? After all, I have numerous handhelds. I've poured money into PSPs and IPS modded GBAs and an Analogue Pocket. I'm a certified Switch soy boy™. The simple answer is that the Miyoo Mini Plus does a lot of things really well. It's a swiss army knife of retro systems. The convenience of just being able to pick it up when I have a spare 15 minutes and resume exactly where I left off in seconds cannot be overstated. It punches well above it's weight, yet it's also cheap, so if I lose it I won't feel particularly bad. Just throw it in your bag and head out, which is what handheld gaming should be about.