I've had my Odin 2 Mini for about 2 weeks now. I'm still getting used to some quirks of Android, but overall I am very happy with it.
It's only a bit bigger and heavier than a Vita, but it does feel more substantial. That's better in some ways and worse in others. I like that it feels more solid and premium (Vita was always a bit creaky), but it does feel slightly less portable. Or, maybe I just need to get used to it. One unexpected thing is that, because the screen is evenly spaced between the top and bottom of the device (Vita is closer to the bottom), sometimes I can't tell which way is up for a second when I pick it up. I'll have to get used to looking for the shoulder button bump outs or the start/select buttons.
Sticking to the Vita comparison, the screen is the exact size as the Vita's. Instead of OLED, it's MiniLED. That's supposed to be a downgrade, but it looks great to me. I have no complaints about the screen whatsoever. One benefit on MiniLED is that it can get extremely bright, to the point where you can play it in direct sunlight. I usually leave it at about 15% brightness, which looks good to my eyes and saves on power. The face buttons are bigger, the start/select/menu buttons are not recessed, and it has L2 and R2 buttons. The d-pad is very similar to the Vita one, and the buttons are all slightly clicky. Overall the controls are great, no complaints.
It does seem like the battery needs to be babied a bit. the Odin 2 Mini has a smaller battery than the other Odins, and I keep seeing posts about battery issues from people who left it charging all the time. From what I've read, the best practice is to only use slow charge, and try to keep the battery between 20-80% charged. Sleep mode is very effective, except when you have a Switch game running in which case the battery will keep draining as if it were running the game. I'm used to just plugging consoles in and leaving them until I want to play, so I'll have to make a habit of being more conscious of this one.
Android is a mixed bag. It's nice that it can run so many systems, but setting up emulators is a pain. I hate Retroarch (I downloaded Robert Broglia's .EMU series for most older systems), and every emulator for newer systems has slightly different settings and menus. Hopefully I won't need to poke around in them too much once I have things set the way I want them. I don't have an existing collections of roms for these higher end systems, so I'll chip away at that over time. I'm using Beacon as the front end, which I actually love. It's very simple and has just a bit of customization. It just works. I'm slowly figuring out what Android apps are useful. In the end, this thing is basically a phone with the cellular parts and camera removed, in a Vita body. It's kind of amazing that it works as well as it does.
To be honest, I haven't had that much free time and most of it has been messing around with settings and trying out different games. I finished Pokemon Heart Gold on my Brick a few days ago, so I'm just now ready to actually start a game on the Odin 2. From what little I've played, DS and 3DS games run great. I do not mind the side-by-side screens at all (I know others have complained about that). I have one screen as big as it gets and the other in the corner, with the L2 button swapping their positions. The one Switch game I tried has worked nearly flawlessly (there was one brief point with a light that flashed incorrectly). I have not tried Windows games yet, but everything else seems like it just works.
I think I'm going to post a bunch of older systems and games in the sale thread soon. Even though it's not perfect, I much prefer just having all of my games in one place, with save states etc. I'm probably going to hold on to my PS4 (mostly for streaming and blu-ray), Switch (I may hack it and give it to my kids some day), Brick (it's just a nice little device for Game Boy/NGPC/Pico-8), and the Odin 2 Mini. Everything else is going to go.