The Shield's library isn't limited to Steam games--you can grab any Android game or app you can think of off the familiar Google Play store. You'll find a curated list of games that play nice with the Shield; some standouts include Grand Theft Auto: Vice City, Sonic CD, and hack-'n'-slash Eternal Warriors 2, all of which have built-in controller support. Problem is, the majority of Android apps don't recognize your controller--and when that happens, you're forced to use the right joystick as a mouse cursor or get your fingers all over the touchscreen.
Using the touchscreen wouldn't be so terrible if it could somehow swivel down over the controller surface, or detach entirely. But as it stands, you'll have to grip the Shield by the top of its clamshell structure, which feels incredibly awkward. It's the equivalent of gripping a smartphone with a miniature ball and chain hanging off it, and no matter how you try to hold or cradle it, using the touchscreen never quite feels comfortable.
Silver lining: If you're starved for great games with controller support, you can make a beeline for the Android's selection of emulators.