The battery life on the 6+ is the selling point of the of this device period. Coming from a Galaxy Note 3 that I loved(and sometimes miss) I've never had a phone with as incredible battery life as the 6+ This phone on average literally lasts me 2 days before I have to charge it and that is amazing! I was concerned that I would be losing out on being able to swap the battery like on my Note 3, but I don't even care anymore. The size wasn't an issue coming from an already large phone and everything else so far has been awesome. Excellent camera, great screen, sound quality, excellent battery life, snappy performance, make this phone a major win. Other than getting used to the operating system(been an Android user since the S3), and the headphone jack being on the bottom(I never owned a 5 or 5S) I'm impressed.
I had a coworker go from the 4S to the 6+ with no issues concerning the size. He took the plunge feet first and has loved it. However I don't think the size is for everyone, but sadly a store demo isn't enough. You really need to use one for 2 weeks first hand to get a real feel for it.
I am a light user, so the iphone 6 actually has great battery life. The standby time is fantastic. I pulled my phone off the charger at 4:50am and currently have 88% battery left. That is leaving location services on, bluetooth on, and wifi on 24/7