A friend of mine has the defender series for her iphone that thing is huge and heavy. On a side note she could roll over it with a concrete truck and it would be fine. She has two twin 2 year old boys and a 4 yr old boy so she needs the protection.
My girlfriend has one. Like Brian said, great if you're that paranoid you're gonna drop you're phone. It's a pain in the dick to get it out of there to charge it and stuff too.
I have the mophie, which is almost as bulky, but for the additonal battery charge, it's definitely worth it, especially if you fuck around on your phone as much as I do.