I just got a Venice Blue quick snap cover, simply because the all gray color of the laptop was quite boring. Plus, I had noticed a few marks of where I had slipped the computer in and out from my carrying case. To me, it was it was a bit more protection for the back of the LCD.
I agree at $40; it is pricey. I tried to go the cheapest way possible. At Parts-People I got a the Venice Blue for $27ish ($19.99 + freaking $7.92 shipping...) shipped. Still quite high for what you're paying for. I recieved it yesterday, and made the computer look quite a bit better, even though the cover is used, very slight scratches and a nick on the top left hinge where you can see the blue coating has been scraped off of.
Is it worth $40? Not really. But it is nice to have that added protection to the back for something many people probably spent $1,000+ on. Not to mention it makes the computer look a lot better, because the standard silver computer is beyond boring. And it does make the LCD display a bit more sturdy to open and close. (Much like Dell's 5100 series laptops.... their hinges are much sturdier when opened and closed.
So -- I wouldn't pay full price for the cover if you have to. But I'm happy with what i got, even though it's kind of hard to justify near $30 for just a piece of plastic.
Oh well, it's much better protected.