I have, and it's ****ing awesome. It feels great actually, a lot better than Otterbox.
See, my job is pretty dusty because of the wind and so I needed a case that wouldn't let dust get in my iPhone, and so I thought the Otterbox Defender would do that, it didn't. After a couple of weeks of use, the screen protector on the case came off... so it became useless for me.
Last Friday I decided to give Liveproof a try since I saw them at Best Buy, and I'm happy with them.
The case felt sorta cheap at first considering that it was a $80 case, but once it's on the iPhone 4, it feels very solid. It also hardly adds any bulk to the phone, compared to the Otterbox cases. I say give it a try, buy it at Best Buy, if you don't like, return it.