I got my GNex with an externded battery - it was VZW "taking care of me" a few months back when I refused to use my Bionic any longer and demanded a GNex. The Manager I worked with had a GNex and said "I am just sending you the extended battery, it'll make you happy". It has. The GNex is the greatest phone I've owned yet and battery life is quite good if you set up/use the GNex properly. Lots of threads here on that subject.
When I recently went to purchase a back-up externded battery for times when I just need more power immediately, I bought this instead:
Amazon.com: Anker Astro 5600mAh Backup External Battery Pack and Charger (Power Bank) with Flashlight for LG: Revolution, Thrill 4G, Optimus 3D, Optimus 7, T-Mobile G2X, P500, GT540, GW620, GX500, KP500, Rumer Touch; T-Mobile: MyTouch 4G, Sidekick 4G
For me, a great decision. No need to remove my case, pop off back, swap batteries, replace back, replace case. It's just instant power - and a crazy amount of it. I can charge my GNex from relatively low battery to 100% 3x on one Battery Bank charge. It's designed and built beautifully. Plus, it charges my GNex 2x as fast as the OEM charger. PLUS, it comes with adapters to charge up my wife's and kid's phones, and other devices too. What a no-brainer purchase for me.
Hope that helps!