This is a cautionary tale for Android users. I don't know if my experience is universally applicable, but take it for what you will (if you want to save time, skip to my conclusion in the paragraph that starts with "Here's"):

I bought the Nexus 5X before the web store even went live. I found some unlinked page on the Google Store and pre-ordered it the second it was available. That's how excited I was. The Nexus 5 had always been my favorite of the Nexus/official Google phones (I've had many), and I was thrilled by the idea of having the same basic phone, but faster, a better screen, a fingerprint reader, and (most importantly) a camera that didn't suck horribly.

Everything was good at first, but at some point (I can't quite remember when) problems started. At first it was minor things, the phone getting a little draggy, some weird behavior. But then everything went south. It got to the point where sometimes my phone would freeze completely. I'd be at the home screen and tapping the app switcher button would start a 60 second wait for it to come up. Then the camera started flaking. I'd launch it, but it would take 20-40 seconds to initialize and let me take a picture. That's a disaster for the parent of a toddler. I can't tell you how many adorable photos I've missed (at least I have the memories). And that's when the camera would open at all! It got to the point where I'd open the camera, and 50% of the time I'd get an error message and the app would close, requiring me to open it again.

Then at some point a couple weeks ago the whole thing came to a climax. I woke up one day and while the phone appeared normal at first, something was very wrong. I couldn't get my quick settings shade to open. I couldn't load the camera at all. I saw emails coming in on the web, but no notifications on my phone. The app switcher wouldn't load. I could launch an app, but pressing the home button would do nothing, so I'd have to always use the back button to get out of anything.

Google phone support had me restart the phone in safe mode, to make sure it wasn't an app doing this, but I had the exact same behaviors. I'm not certain one of my apps didn't cause this, but I can't think of what might have.

In the end, I had to go to great lengths to grab any files from the file system that weren't backed up anywhere (mostly audio recordings of my son that I made with a voice recorder app), and I did a factory reset.

Here's where my warning to other Android users comes in: set up your Android phone as a new phone. I love Google's backup feature, and I've used it on other phones, but in some cases it seems to be causing some problems. I don't know if it's pulling something down from an old phone that it shouldn't, or what, but it's looking more and more like this is a problem feature. When I set my phone up again, I chose to set it up as a new phone. I'd made a note of all the apps I wanted to reinstall, and went through the annoying process of getting everything set up again from scratch.

It wasn't fun, but now I have a phone that functions as I had hoped it would. I never expected high-end performance from a mid-range device, but what I was getting was 10x worse than my mother's old Moto G. Now my phone works as well as I'd hoped, and I'm getting those great shots of my son that I was hoping for.

Sorry for venting, but this has been a long ordeal for me. I went through months of this annoyance and I'm happy to be done with it. It's possible that this problem will return, but I'm hopeful that I have it licked.

What say the other Android users here? Do you remember which option you chose when setting up your phone?

This sounds so much like the old Windows 'nuke the hard disk from orbit and repave' cycle.

I'm still using the same backed up/restored phone data on my iPhone that I first used on my first iPhone in 2008

I have had people say not setting your iPhone up as a new phone leads to problems, I've not had a problem with it.

Last time I tried to use the backup/restore approach on my Nexus 5, half the settings/data where missing

My multiple iPhone upgrades have also worked fairly smoothly. Backup old device, connect new device. Prompt appears to restore from backup. Click Proceed, let it work away.

Same for warranty replacement devices restored from backup. I almost always restore from iTunes via USB on Mac. Not a fan of iTunes in general, but it does 'work'.

Just did this a few days ago for an AppleCare+ warranty replacement of iPhone 6s with a slightly sticky Home button. Called Apple, arranged for replacement device to be shipped to me. It arrived here the next day. Backup old and restore to new, then handed the fully configured replacement iPhone to spouse. Everyone is happy.

Pack the old (factory reset) iPhone in the supplied return shipping box and call for prepaid pickup. It will be heading to Apple tomorrow. UPS just picked it up.

Isn't that the sign of a "mature" operating system?

To be fair, I can't imagine that this is a problem that all Android users have. I think it's more likely that a strange collection of circumstances that led to this. I do think it's a problem that Google needs to look into, so maybe I'll submit it.

Prior to this, I'd used the backup data for every Nexus device that's supported it. This time it did not work.

My own Nexus 5X, also purchased when they were the hot new thing, has been doing better than yours. It does occasionally get janky, and a reboot will generally fix it. I do that maybe once a week and it's fine.

I just purchased another Nexus 5X to be my daughter's Very First Real Phone. (Using Google Fi to get it on the cheap, and it's unlocked so easy to switch later.) We'll see how that works...

I've also used a Nexus 5X, so I have some experience with it.
I've been an iPhone user for the last 3 years. I've bought an iPhone 5 a couple of years ago pretty cheaply because somebody else payed half of it. I had been using that phone for 3 years, but then it started to act up (mostly battery issues).
So I got rid of it and was reluctant to shell out 860 euro for a new iPhone 6s. (read: I couldn't justify it to the household budget ) So I was like: "heck, those Android phones probably aren't that bad also, after all, more than half the planet uses them". So I bought a OnePlus X. This phone worked fine for 6 weeks, but then the WiFi started to act up. It would connect, but no data would get transferred. This obligated me to use my 4G connection all the time. I tried everything: factory reset, forums, and eventually even sent it back to OnePlus. It returned a few days later with the tech report, saying "nothing was wrong with the phone". So I then decided to get rid of it.

Still reluctant to shell out the cash for that new iPhone, I then bought the Nexus 5X. (which was 100 euro more expensive than the OnePlus X, but still only half of what I would pay for a 6s)
I must say: I really liked this phone. I just found it a tad too big. I didn't get any of the issues that you had, though. And I was thoroughly surprised when I learned about the Google backup feature, that was able to completely reinstall the 5X with the apps that I used on my OnePlus X. Still not a good as the iPhone backup though, but acceptable.

But after 3 weeks of nagging my wife that I still found the screen too big, she told me : "then get that damn iPhone!"

She didn't have to tell me twice.

Two hours later I held a shiny new iPhone 6s in my hand. I've been happy as a kid ever since. The backup utility of iTunes neatly put my last image back onto the new iPhone 6s, and walla: everything was back, up to my texts that ended the day I stopped using my iPhone.

This time I'm here to stay, I don't care what happens. An iPhone is expensive, yes. But it simply works, all the time, every time. There's a lot to be said for that. I am happy that I've at least tried Android though. To be honest: it wasn't half bad. I would even call it pretty good, but it's just not as stable as an iPhone. And I've come to an age where things have to "just work". The days I could get some gratification of spending hours and hours troubleshooting some problem and then finally, at 4 AM in the morning finding the solution are long gone. I've got a family now, and a life. My time have become too precious.