Getting RetroPi installed on the SD card was child's play. Making it usable has been something of an adventure.
The first hurdle to be cleared was setting up wifi. That was actually pretty simple--once I connected a USB keyboard to the Raspberry Pi. Wifi was up and running in a matter of minutes
Next hurdle: loading some ROMs onto the card. I thought I could just put the card into my Mac, find the appropriate folders, and then copy the ROMs there. When I discovered I couldn't do that, I was disappointed. Then I tried using the prescribed method and was floored by how simple it was. All I had to do was type smb://retropie in to my Mac and click connect. From there I could easily find and fill the appropriate folders with my ROMs. So simple, even I could do it!
Then came the controller. Ultimately I plan on using my massive X-Arcade controller with this rig, but for now I was hoping to use just my Mayflash controller. It's a lovely controller and all, but not useful enough during setup. I then hooked up a PS4 controller, hoping it would allow me to do more. Sadly, I got the same result: great controller, but I have learned that you really really need a standard keyboard to be able to do certain things. I was hoping to avoid that. Alas...
Rather than continue using that PS4 controller, I decided to order a couple of SNES-style controllers from Amazon. That way I could keep them with the RasPi at all times. They're due to arrive tomorrow. I should be able to get back to this in a couple of days or so.
Side note: A couple of years ago I tried this same sort of project using RetroPi's predecessor, PiPlay. I could never get past the initial installation. That I have made it this far is a testament not only to how well though out the system is, but how good the setup docs are. Very happy about the progress I'm making!