I don't have a Raspberry Pi, but my friend brought one over to show me and it ran fine but had terrible (I am assumimg were default) graphical settings. The picture was stretched 16x9 for some systems, and all seemed blurry (not point sample scaled). I do want to get one and try it, but haven't taken the money and time to do so yet.
It's hard to argue that hooking this thing up and playing it will be a lot different experience than plugging in a AC, HDMI, a controller, and a cart/cd. The steps to use the Retromegabox will be a lot different for non-pc/emulation enthusiasts.