I wanted to ask few questions about the console before buying.
The price tag for it is quite high (which I understand why), but there are other cheaper alternatives which seem to be as good. For example, Odroid Go Advance which is 2.5X cheaper, the GPi case for Raspberry Pi, GPiMate, and many other in the price range of €35-60. And then there's a tonne of Chinese no-names with hundreds of games which cannot be updated and generally shouldn't be considered an option, but if it's just for a few games then it's totally worth it.
How do you choose and why would you prefer RetroStone here?
And a more general question about the 4 buttons. I'd like to play Mega Drive/Dreamcast games which make use of 6 buttons, but having only 4 buttons on the RetroStone how do you guys deal with that?
Questions before buying
Re: Questions before buying
Regarding GPiCase and Odroid, note that those are unassembled kits. So they save cost on assembly and quality test. Initially when I launched Raspiboy it was a 49-59€ so similar to them. But after you add lot of small features the manufacturing cost adds slowly but surely 
Regarding GPiCase I don't really see why it had so much hype actually. Probably because they copy-pasted gameboy design.. But it has lot of downsides:
- 2.8" screen only 320*240 resolution (RSN2 is 3.5" 640*480). And the screen connects with pogo pins to the pi zero. I used pogo pins in Raspiboy, and I know it's not so much reliable, mainly because it's not designed to make permanent contacts like this. So expect potential issue with the screen. Though it might work very well without any issues.
- No USB connectors, so no multiplayer
- No HDMI connector out available.
- No rechargeable battery. I how many AA batteries are you going to waste? And how long 3AA batteries will keep the system running?
- Pi zero not included, so you have to add it.
- Bigger than RetroStone in dimensions.
- No ethernet
For the Odroid I haven't seen it before. It looks like a decent option. Screen is 320*240 though. And it doesn't have HDMI out, and only 1 USB.
Also I think they are a bigger company so they can have better prices for components and lower margins.
About chinese pre-loaded systems, those use much cheaper hardware. Small processor, memory chips cost much less. And as system is simpler, they have only 2 layer PCB which cost much less too. They use low quality screens, speakers. Also the injection molded cases are smaller, they use cheap plastic, thinner walls and so on. So overall the quality feels much lower. Though they do work for their intended purposes. They even have some advantages like instant boot time. RetroStones are full linux computers, so they offer much more possibilities.
Overall as you say there are some options which are 2.5X cheaper, even 8-9X cheaper if you consider the small chinese consoles. But the price itself (135€) is still not so expensive for a game console so I wouldn't let the price difference make me choose one or the other. I think what is important is that you choose which meets the best what you'r looking for, and it could very well be a small chinese one!
Regarding GPiCase I don't really see why it had so much hype actually. Probably because they copy-pasted gameboy design.. But it has lot of downsides:
- 2.8" screen only 320*240 resolution (RSN2 is 3.5" 640*480). And the screen connects with pogo pins to the pi zero. I used pogo pins in Raspiboy, and I know it's not so much reliable, mainly because it's not designed to make permanent contacts like this. So expect potential issue with the screen. Though it might work very well without any issues.
- No USB connectors, so no multiplayer
- No HDMI connector out available.
- No rechargeable battery. I how many AA batteries are you going to waste? And how long 3AA batteries will keep the system running?
- Pi zero not included, so you have to add it.
- Bigger than RetroStone in dimensions.
- No ethernet
For the Odroid I haven't seen it before. It looks like a decent option. Screen is 320*240 though. And it doesn't have HDMI out, and only 1 USB.
Also I think they are a bigger company so they can have better prices for components and lower margins.
About chinese pre-loaded systems, those use much cheaper hardware. Small processor, memory chips cost much less. And as system is simpler, they have only 2 layer PCB which cost much less too. They use low quality screens, speakers. Also the injection molded cases are smaller, they use cheap plastic, thinner walls and so on. So overall the quality feels much lower. Though they do work for their intended purposes. They even have some advantages like instant boot time. RetroStones are full linux computers, so they offer much more possibilities.
Overall as you say there are some options which are 2.5X cheaper, even 8-9X cheaper if you consider the small chinese consoles. But the price itself (135€) is still not so expensive for a game console so I wouldn't let the price difference make me choose one or the other. I think what is important is that you choose which meets the best what you'r looking for, and it could very well be a small chinese one!
Re: Questions before buying
Thanks a lot for such a comprehensive answer!