The LifeProof case's claim to fame is that it's the toughest iPhone case you can buy.
Indeed, this thing is no 10-pound weakling. The company says that the LifeProof case is waterproof, shockproof, dirtproof, and snowproof, yet it's slimmer than virtually all the other tough cases out there, with an integrated clear plastic shield that protects your screen (and, obviously, seals your iPhone into the case). Also, according to LifeProof CEO Gary Rayner, the case cost $2 million to develop.
After using it for a while, I have to say it does strike an excellent balance between sleek looks and toughness, and it's the case I use most on my phone. And yes, it totally waterproofs your iPhone 4 or 4S.
I've had it on my 4S off and on, mostly on, for the last five months and it's held up very well and only has a few small dings from when I dropped my phone on the pavement. I took it underwater in a pool -- yes, you can shoot video underwater, though you need to start shooting the video before you go underwater because the touch screen doesn't work underwater -- and dumped it in a snowbank and my iPhone came out no worse for the wear. This case is drop-tested for 2 meters (6.6 feet) and is rated to go underwater to 2 meters. It also fits the iPhone 4.
As impressive as the case is, there are a couple of small issues worth highlighting. First, the headphone jack is recessed, which means that with most headphones and earphones the plug simply doesn't fit and you'll have to use the included headphone adapter. There's a good chance you'll lose the adapter or the little plug that you screw into the headphone jack to keep everything nice and watertight. LifeProof offers replacements (at a ridiculous price of $19.95), but the adapter is a nuisance to deal with. One way around the adapter is to simply use wireless Bluetooth headphones, but some people don't like the sound of Bluetooth headphones.