I wanted to get something like this as soon as I got my Nokia 6300 in hand.
I have seen my friends putting their mobiles in little plastic covers like this one and I thought it was a great idea.
The covers vary in designs, some have different areas exposed in different ways, be careful to look for one that will allow you to press the buttons easily. I only really wanted something that would protect the screen and take the first blow if i dropped the phone.
The transparent plastic on the screen can sometimes allow for "sweat" to develop between it and the screen, so you still have to dismantle this cover to clean it from time to time. Some dust also manages to get in there and it gets trapped.
I was also a little bit scared that the plastic may allow for some friction with the screen, therefore creating scratches, but I saw none for the time that I had it.
One of the major draw backs with these covers is that its very hard to find one of good quality. If you pay for a cheap one on Ebay, like I did the first time, you will get one that is made of very light and abrasive plastic, and it will last for about 5 minutes before it breaks. Mine broke in 3 minutes... They also dont clip in nicely, so they come off, which defeats the object. Many times I found it unclipped in my pocket and it nearly scratched the screen. They are very cheap but a total waste of money.
The more expensive model I got from the high street was made of far better plastic, polished, smooth and sturdy. The cover clipped around the phone a lot more securely than the cheap version. I can truly say, you get what you pay for with these covers.
In the end I gave up on them because I was not comfortable with the plastic laying over the smooth screen. They clip nicely but there is still room for little friction and that can cause damage on the screen, especially with the 6300 Nokia.
I know protect my phone in a pouch, and I find that a lot more "gentle" on the phone.
By all means try this product, but do go for a sturdy original version.