John Gruber posted a nice write-up of iPhone cases (or not) and personal preference. I can tell you mine: case, always, and rarely Apple’s. Everyone is unique, but in the interest of sharing, here’s why I do as I do.
I use a case because I dropped my first-gen iPhone the first weekend I owned it. Just a scuff, but a lesson learned: iPhones are slippery, shaped inorganically, and in cadet-sized hands also dealing with children and pets, they can and do go flying. And, frankly, cases work: in all the years I’ve owned an iPhone, I’ve only broken the glass once—when my phone fell out of a broken case that I was preparing to replace.
I’ve generally bought third-party cases because they’re slimmer than Apple’s. As long as they have a lip around the front edge, to protect the screen, they’ve done me well. There are downsides, though: the fit can be off, and they tend to break down more rapidly. I go through a plastic Case-Mate Barely There every six months or so on my current 12 mini. Meanwhile, the Apple-issued case I have on my 2017 iPhone 7 (which I still use for audio and games) is still hanging in there.
For my new, plus-size work phone, I got Apple’s case, and it’s really nice. I love its tactile qualities. It doesn’t seem much larger than my Case-Mate, either. I may get a second Apple case when I upgrade my mini. (I’m also eyeing some of the makers Gruber links to.) Whatever I buy, though, I’ll have a case on it promptly.