Paige is spot on. (I don't think it was a harsh reply either. this is an opinion thread, and he wasn't being antagonistic, just anecdotal).
Typically the Pi is better suited for smaller projects like mini-cabs and bartops.
I meant harsh to the Pi not the OP

You said it yourself they are good for smaller projects which is what I said above

I had a go with a Pi just for the hell of it to see what it was like more than anything and it emulated all the classics perfectly well which is exactly what I would want it to do if I was building a cab with it.
No it cant play really heavy games but I would not expect to be playing those on a small machine either.
The point is it has a place and to dissmiss it as "the worst possible thing available that anyone is actually powering their cabinets with." is IMO harsh.
The downside to the Pi is the shear amount of work required compared to the gain.
Where as you can set up a low end PC running a FE and mame with all the nice box art etc quite quickly doing the same for the Pi takes ALOT of time.
You certainly cant just dump a full rom set on it and expect it to work you need to rebuild the set, sift through all the stuff that wont work, take ages transfering all the files over and then start looking at the nice bits.
Not to mention the whole powering up and down bits etc.
Admitidly though once you have a good set up it is very quick and easy to replicate and reproduce.
But yeah for small projects I think they are wonderfull, But they dont hold a candle to a PC or JAMMA set up.