After purchasing an Otter BOX defender Series case for my BlackBerry Curve 8520 I've got to say it's well worth the money as it feels a lot stronger than the generic Hard case that I bought.
The clear cover that goes over the keypad protects it from getting dirty and bits getting in to the phone through the keypad, The Hard shell feels a lot stronger than the the generic hard case that I was using previously and I like the clear protector that goes over the camera lens to protect it from getting scratched and the silicone outer skin is a lot stronger than a normal form fitting silicone skin and I like how their is a flap that covers the Micro USB port and the headset socket to protect them from letting dust into the phone. The only open parts of the case are just the microphone on the bottom of the case and the speaker on the top of the case as well as the handset speaker for non headset or speaker phone calls. The provided belt clip holster is a nice addition to the package as the Otter BOX makes the phone very bulky and heavy. The plus point of having a heavier and bulkier phone is that you are less likely to drop the phone compared to an un protected Curve as they are very light.