My GF has 128 GB in there and is using a large part of it on photos and music - I don't actually have one in there right now, though I've used it to transfer photo's from my camera to the net before. A phone with 16GB memory and a 128GB card is far, far cheaper, and more versatile, than buying a phone with 128GB memory.
Battery wise, though, yes, I've switched a bunch of times - using hers to access my phone when my battery's (almost) dead, charging the other one (we bought one SIM card while trekking in New Zealand, for example, and I used both batteries in that phone, and hers only for Wifi and charging battery). Also, I've had a few crashes where the phone didn't respond to the power button and only removing the battery got it to turn back on.