Actually, face unlock is mightily impressive. The improvements made to it since its first iteration have made it a pretty viable unlock option. I've recently started using it on my Nexus 7 and on my HTC One. It's faster than entering a pin, and slightly more secure now that it can't be fooled by pictures.
You can also scan your face multiple times to improve the facial recognition. So you can do one with and without your glasses, you can do one in optimal lighting and in non-optimal lighting, you can do one with and without a hat on, etc.
And you'd think this would muddle the facial recognition but it doesn't. I've tested it. Sure enough, if it isn't your face behind those glasses, it doesn't accept it.
It's also surprisingly fast, especially on the Nexus 7 where the front facing camera comes on right away the moment you wake your device (versus the HTC One that requires a swipe up first from it's main lock screen). It can unlock in less than a second if it's aimed directly at your face. In fact, it unlocks so fast that I sometimes wondered whether it was even really reading my face or not, but again, sure enough, if it's not your face, it won't unlock.
It'll also default immediately to your secondary unlock option (pin/pattern) if it fails the first time or detects that it's too dark to see you, which is nice as this only marginally interrupts you. A note on dimly lit situations: it's actually amazing how dim it can get before it will fail to recognize you. I tested it in a room with all the lights off except for the monitor -- very low light, my face is barely visible, and it unlocked.
Very impressive. If Google is continues improving this feature and as front facing cameras improve (go OEMs), this can only get better. It's a shame no one really talks about it anymore.