Ditto! First 4, then 5, now 6.
I find the phone battery only lasts a day, but I use it a fair bit (though rarely as a phone!). Much the same with previous iPhone 4 and 5 when I first inherited them, though the 5 had deteriorated so I replaced the battery (DIY) and got up to 2 days for a while.
My wife has a refurbished one, passed to her by a nephew which had badly cracked screen and broken wifi, which had been repaired while-you-wait in China for about £30 including new battery! Hers lasts about 4 days between charges, but she does use less than I.
It is truly fantastic having a pocket computer that give me internet browser, Discourse Hub reader, email access, calculator, zoomable map, GPS, satnav, bank security device, music player, music controller, translator, language teacher, currency converter, e-book reader, voice recorder, notebook, alarm clock, cycle/walking statistic recorder, noise meter, light meter, camera, magnifier, video communicator, messager, remote computer controller - and it can make phonecalls as well!