I had a Nexus 7 (2012) and my wife has a 3 yr old iPad 2. The Shield Tablet hasn't blown me away with battery performance so far. Standby is much improved when I set WiFi to turn off when the tablet is sleeping, at least in my recent several day test, but even then with light usage it's only lasting 2-3 days. The screen definitely sucks a lot of power so I can take a 15 percent battery hit by using it for note taking in a couple of meetings, which means having the screen on for maybe 45 minutes cumulatively.
I haven't done much gaming or anything else demanding on the processor (maybe Dabbler makes it work a little) so can't comment there.
All in all it's superior to my aging Nexus 7, but can't touch the iPad, at least for standby time.
Now I've just downloaded the firmware update, so check back in a week!