Got my hands on the RG557 a few days ago, posting thoughts

>good
Feels very comfortable in the hands.
Anbernic finally released a handheld that can be called powerful. Havent tested a lot but there were a few PS2/GC games that I was able to play at 3X resolution, either at fullspeed in high perf mode (like Silent Hill 2, Manhunt 2's "uncut beta", Sonic Heroes GC's first two stages, Yakuza) or mostly there with some dips to the 50s (the first track in F-Zero GX, Smash Melee, Final Fantasy Crystal Chronicles).
Comes with emulators preinstalled
OLED panel
>eh/unsure
RGB lighting is better than the 556 but I turned it off anyways.
Face buttons are a bit stiff but usable
Not sure where the performance cap is. Still havent tried Drakengard 1 which brings weaker ARM devices to their knees even at 1x res.
Apparently AnbernicAI can be used to do machine translation for games, but it seems to require a terms of service agreement so I havent touched it.
Apparently some models being shipped out are lemons with stuck buttons or bricked.
>bad
Plastic feels very cheap compared to the RG40XXV, even with the white model my gross greasy hands still left smudges.
Downfiring speakers
Battery life is mediocre at 5500mAh
Analog triggers dont seem to have a lot of travel.
Too damn expensive at MSRP
Analog sticks too small. Fine for third person games but not ideal for fine-aiming in Timesplitters 1 personally.
Some games like Shadow the Hedgehog at 2x on Dolphin or GT4's Seattle Circuit on AetherSX2 at 3X still have pretty bad slowdown. Not sure if this is a problem just with Mediatek drivers or if this on the emulator.
Fan gets fucking loud on high perf mode
Apparently some android gachas refuse to work with it due to being a "rooted device"

I'd never pay full price + shipping, but I paid $309 CAD for it on Aliexpress with free shipping. Worth a look if its on sale and you want a capable 6th gen emu device, avoid if you really want to do Android gaming + switch/wii u emu.