I've had my 6+ since the day it came out and have been through 4 cases: Spigen Slim Armor, Ballistic Urbanite, Zerolemon Zeroshock battery case and now the Mophie Juice Pack.
Well I just bought the mophie today for $20 off (Best Buy has them for $80 before taxes today) and like it the best. I had it on my iPhone 4S when I had that. The Urbanite and Zerolemon I've only had for a few days. The Urbanite wasn't bad but I didn't think it was worth $27. The Zerolemon battery case didn't even work at all. I didn't mind the bulk but since the case came defective and couldn't charge my phone it was just a larger Otterbox case to me and so far the customer support at their website has been terrible so I'm returning it and never purchasing from them again. I found out they also tell ppl to dish out positive reviews for them for their crappy products. The case wasn't even bad, good quality but it was useless and came broken and I'm just one of many ppl it has happened to. The mophie case doesn't offer as good protection but it looks better, is slimmer and actually works.