I rarely game on droid now but occasionally on trips I will delve in, so do keep an eye on the scene a little (Not Shield specific).
3ds emu Azahar is a recent one I have seen chatter about, aPS3e/RPCS3 brings some games, NetherSX2 patch for PS2. My SD845 chip can run some stuff but I genuinely stay below PS2. With the SD elite stuff bringing so much power most of the hype has moved on to PC/Steam gaming apps like Game native and GameHub etc.
I'm not sure where the Shield is at in the current landscape!
Russ of retrogamecorps is usually on point.