I've had my EXT since March 17 and I'm on the 3rd battery-23000 miles. Both faulty OEM units were replaced under warranty but required visits to the dealership and alternate transpo to work, and my time to do so. I will say that other than the batteries, my car has been flawless in operation, so compared to the horror stories I read on this forum I feel lucky that I got a good one.
I was lucky enough to get 2+ years on my OEM battery. But I replaced it anyway before the winter because I knew it may not make it though the cold weather.
Consumer Reports top rated battery for this size is a Duracell Gold which I purchased at Batteries Plus.