My husband and I swear by our Lifeproof cases (they are owned by Otterbox now). My phone has been in the toilet, pool, lakes, and rivers, and dropped countless times, in the dirt and on hard floors. My husband fell while pulling the boat out of the water in the lake, landing on his phone while it was in the belt clip… rinsed the phone, and it's good to go.
Here's hubby's online review:
http://hunting.about.com/od/huntprod...lip-review.htm
Be aware--there are cheap fake knock-offs. My sister had a fake that she thought was authentic when she bought it. It passed the water test but sound was very muffled. After trying other cases and not liking them as much, she bought another cheap Lifeproof knock-off and is happy with this one. But, of course, no warranty on the fakes.
No, the phone is not guaranteed by using a Lifeproof case (although I think they sell insurance now as an add-on). But they have very good customer service. My case was replaced twice.… first for the screen sticking to the front glass, and another time for scratches. I still would not have anything but Lifeproof. Plus, the kiddos love getting underwater photos and videos taken, so it's a lot of fun.
BTW, both our phones are bought outright (not subsidized) so protection is very important to us.