These clear acrylic cases are much of a muchness nowadays... I mean how can someone justify paying more than a pound or two for a plastic case? The point I am trying to make, is that even with something this simple you really do get what you pay for.
I have had many cheap (well three) plastic cases from ebay. Two for mobile phones and one for my son's iPod classic. They all cracked within about a month, very brittle and thin.
The Griffin iClear is very well made. Quite a thick plastic and finished off nicely with smooth edges. To fit your iPod into the case, you must first apply the bonus screen protector. This is just a thin film of plastic that adheres to the iPod screen with static, no glues, so no marking. You don't get this little bit of plastic with the cheap ebay ones. Then you place the front portion onto the front of your iPod. Turn the iPod over and snap the back into place.
Yes, this is a simple case and £10 (my purchase price) might sound like a lot of money. However, this has lasted five months, protected the iPod classic and has been worth every penny. The quality really does show.