I have 2007 MBP 17" and Speck see through case around it.
+ got no scratches on body
+ resistant to dirt and throwing smth on top of my laptop
+ highly customizable, if u want, put stickers, then remove them or just put smth inbetween the cover and actual mac. I've printed on a rare superthin japanese paper an artwork of mine and company logo I work for. Looks amazing. Everyone pays attention and says its cool
+ does not overheat
- adds weight
- adds thickness
- does not let you open screen that wide as without the case around it.
P.S. And to be more freakin' protected:
1) Have screen foil, which is cool, because I do stick my fingers and sometimes point with a pen (LOL) and I spit when I talk, too

2) Have a silicone white transparent keyboard protection. I eat while I work

and my keyboard looks like new. And the silicone is easy to wash with kitchen detergent time to time. If you look my keyboard silicone towards light,

, Alt, Ctrl, Shift and letters ASDF JKL are super polished.
3) And then there's two parted palmrest transparent sticker together with a sticker atop of trackpad and its button. Bought in ebay. As I wear sharp chains around my wrist and sometimes scrape the laptop while rendering, to fasten the process

then I believe it has protected well enough.
4) Around the whole laptop there's a neoprene sleeve to carry it around in an even more softness and care.
Summary - I invested into all this protection crap some days before my actual BTO laptop arrived and I believe it will increase noticeably second hand price of the kit once and if I will ever need to sell it. I believe it will remain mine forever, because it is simply amazing computer!!!
__________
MBPro 17" (2007), 2.4Ghz, 4Gb, 500Gb 7200rpm, 1920x1200 hi-res.