I have all the Shield devices... Shield Portable, Shield Tablet (original w/stylus, 32GB and 4G), and the Shield Console (16GB version, 64GB SD card set as adopted storage, and 500GB external USB hard drive). I think they all work great for emulation. I use the portable to play several MAME games (old stuff I like such as Gyruss, DK, DK Jr., Gunsmoke, etc.) plus fighting games like MSH, SFII, Darkstalkers and SNES games... Same thing for the console, I play Super Punch Out frequently on it. Plus I've used the portable and the console for streaming games from my gaming PC which has worked pretty well. The most surprising experience was I tried out Arkham Origins streaming from my home PC (using the Shield Portable), across the internet, to the public WiFi at work (which was very iffy at the time) and it was still perfectly playable. I actually beat some of my records while streaming, so that should tell you the timing was still right on.
So I guess it's not necessary to point out I'm a bit of an nVidia fanboy...
Oh, forgot to add, I also use ScummVM on the tablet to play things like Full Throttle, The Dig, Sam & Max, and the Indiana Jones games...