I have used Google Photos for all my pictures because of the massive amount of photos that I have. My old 16GB iPhones could never hold all the photos and iCloud wouldn't let me offset them back in the day to cloud only. So I went to Google Photos.
I have 96,341 photos and videos in Google Photos - about 93GB even at Google's compressed rate.
I got my 8+ September 26 2017, and I've taken 785 photos so far as of Oct 19 2017. And this takes up 2.6GB on my iPhone right now. Since I have a 256GB iPhone, I will probably not delete photos anytime soon.
Now, not all these are photos. I do a fair amount of screenshotting, etc... I use my phone as an accessory to my brain to help me remember stuff. And I use DayOne to journal - so that and Google Photos reminding me of photos on this day in the past, it's a lot of fun to look back on them.
For my family's birthdays, I can type in the person's name and get access to all the photos I've taken over the last 20 years. We have had a tradition in my family, I post a ton of photos throughout the years of X person on their birthday in our family chat. It's a huge hit.
Like others stated, with Google's increased searching algorithm support, I can search anything and it will find all the photos I've ever taken with that subject -- calculator, fire, red, bird, cellphone, screenshot... And I can quickly type in any day in my past and have a REALLY good idea of what happened on that day specifically because of all the photos I take. I take about 800-1200 photos a month (more this month because I just got my 8+).
I store my photos in my iTunes as a backup. Google Photos are backed up on 3 different hardrives in 2 different locations several times a year. OneDrive and Amazon Prime Photos also backs up my photos and I turned on iCloud backup with my 8+ purchase (I got 200GB for iCloud family for $3/mo).