[everfree]

I've tried an emulation box with RetroPie and the controller drivers are hard to find (at least for xbox 360 wired controller), the controls are difficult to set up (requiring manual edits to config files, which you need to plug a keyboard into your rpi to do), the controls are glitchy after you set them up, and the interface is not intuitive at all. If someone has a better idea for pi emulation I would really like to try it.

[ashark]

Use Lakka, not RetroPie. Use a PS3 controller—360 would work, but its dpad is terrible, and both work with no extra effort. PS3 even works over bluetooth with little to no fiddling. Only trick is you have to find the "right" roms (those blessed by various projects that record hashes of accurate rom dumps) or Lakka/Retroarch's auto-rom-scanning won't work, though you can still select roms manually. Small price to pay for the level of "just works" that Lakka provides.

[EDIT] links:

http://www.lakka.tv

Which romsets to use for various systems:

http://libretro.com/forums/showthread.php?t=6661