I switched to my first iphone nearly 3 years ago. I still have the same phone. At the time, I got it mainly because I wanted a waterproof phone. I've never had a phone get damaged by water, but I didn't want to worry about it either.

Unlike any previous (Android) phone I've had, it still feels just as fast as it did when it was brand new. To date, the longest I've had any phone is around 3.5 years, and in 2 out of 3 cases it was due to hardware failure. Even if this one were to die tomorrow I'd definitely get another, although I think it's more likely that this one will last at least another couple of years, at which point it will be ~5 years.