We'll, my RG351P arrived about a week ago. I've not had much spare time, but have started to get it set up.
Stream of consciousness incoming...
Initially played with it as it came, just to check it out. Quite impressed with what it can cope with. Dreamcast Crazy Taxi seems a step too far (you can play it, but it's a bit low on frames), but PS1 plays perfectly, and you can get good enough performance from Sega's Atomiswave arcade platform (Knights of Valour and Dolphin Blue are just about a pass).
Bought a decent micro-SD to replace the dodgy budget one it came with, and installed AmberElec. I've ditched the 10,000 games it came with, for the most part, and have gone with my own collection. 10k is just too many to navigate!
The promised WiFi dongle didn't come in the box: I just bought a compatible one from Amazon. Not tried it yet though.
I've mostly been playing Super Bread Box on the C64. Bought that from itch.io a couple of months ago: it was made in 2013, and kind of blows my mind how good the C64 can still be with modern game design sensibilities applied.
It's reasonably comfortable to use, particularly when you're not using the shoulder buttons: not overly keen on the positioning of those. It's not as good as a proper PS controller for something like Tony Hawks 2, and I suspect it'll not be great for Unirally on the SNES either. Perfectly good for most 16-bit stuff, though.