I had the Otterbox Defender for my 3G and loved it. It was a tank, and survived an ambulance driving over it once.
I now have an iPhone 4 and bought the defender case. The defender case for the 4 is more streamlined than the one for the 3G(S), but I do not like the design as well. I found the rubber outer cover comes loose too easily for my tastes, and its texture makes it very difficult to get out of my pocket.
I have since purchased the Otterbox Commuter series for my iPhone 4, and am very happy with it. While it does not seem as robust as the Defender, it still seems to offer very good protection, and I'm very happy with its fit and finish. Furthermore, it has the rubber on the inside and a polycarbonate outer shell, so it slips into my pocket easier, but there's still enough rubber exposed so that I can hold it securely.
You don't say which model iPhone you have, so I will summarize:
The Otterbox cases are excellent.
If you have an iPhone 3G or 3GS, I'd recommend the Defender
If you have an iPhone 4, I'd recommend the Commuter