When the Iphone 4 first came out many users were experiencing call drop outs and loss of signal when holding the phone due to the Wifi and mobile 'aerials' being the silver metal part around the phone and holding it a certain way joined the two and meant you lost signal. So to combat this Apple offer free cases.
This was the one I chose.

It is simply a see through plastic cover that goes around the back and sides. And it does seem to do work in stopping signal being lost. It has holes in it so you can access the volume buttons and connect the charger without having to remove the case. That being said the case can be easily removed for cleaning. Being see through bits of dust and fluff will show up against the background of the black phone meaning frequent cleaning it need.
Its other benefit it its very hard wearing. I've lost count of the number of times I've dropped my phone and the case has come to the rescue. Although the case its self is now chipped in a number of places the phone has survived unscathed

Looks wise it doesn't do a lot for the phone, in fact it takes away the stylishness of the iphone 4 and makes it look more clunky. But I don't think it's designed to be the most aesthetically pleasing case, it's
meant to be functional rather than make the phone look good.

The case can be picked up for under £5 on amazon and eBay so if you want a case that provides more protection for your iPhone I highly recommend this case