I run mine naked about half the time, in a tpu case the rest of the time. I have two very small nicks in the silver trim around the sides, and a little scuff on the home button. I've had the phone since a few days before launch, though I did replace the battery cover recently because I broke it taking it off one time. Always have an SGP Ultra Nano screen protector though.
I'd say most people should have no problem running with no case, but it may not be the best idea. I love the feel of the naked phone and frequently use it like that, but I have dropped it from about waist height a few times onto hardwood floors or low carpet.
The phone feels nice enough that you do keep a firm grip on it and treat it like your precious, and if you're careful to keep your pocket empty you won't scratch it there. But because it's so slick, it can slip out of your pocket when sitting, or off of a surface you've set it on. Plus if you have anything else in your hand, it's easy to forget to keep a firm grip on the phone.
I've now got a Cruzerlite tpu case that looks and feels awesome, and like any tpu case removes in seconds with no wear. The material provides a great deal of protection for its minimal bulk, I've accidentally thrown my old Droid RAZR onto asphalt and concrete on separate occasions with just a slim tpu case with no harm done. Recently, I've been carrying with that case for peace of mind, and taking it off on occasion to use it, or if I'm at something rather formal.
But I see no need for something more than tpu, like an Otterbox, for the average person. If you're particularly clumsy, if you frequently skate, run, bike, 4 wheel, hunt, etc with it, or have a fairly physical profession I can see it, but for the rest of us it's overkill.
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