Had my Miyoo Mini for a couple of weeks now and it's an absolute game changer.
Something that small, that quick to wake up, that runs so much and somehow makes everything look so gorgeous. I've been playing retro stuff on a Vita for a couple of years - but everything looks sharper and cleaner on this screen. No temptation to put any ropey filters on things, it just looks right.
Just the sheer convenience of it. Took a quick trip to Disneyland Paris at the weekend, and being able to whip out Chrono Trigger in the queue for a ride, then nip it back in my pocket right before boarding (in the case...apparently the screen is pretty fragile), was awesome.
While I'm here - hard recommend the Rosy Retrospection mod of GB Tetris. Adds hold (with Select) and hard drop, and therefore improves perfection.