I use a Booq Mamba case which opens fully and i just use it as a "pad" for my laptop when I open it somewhere dodgy. Cases... I wanted one but decided rather not to.
I think that the case of the MBP is used to dissipate heat, so if you put a case on it, you might cause it to heat more, causing the fans to spin faster so more noise etc.
My last mbp was bought in october 2008, it's still shiny even if I never used a case. But in my laptop bag I have a reserved place for it, padded so it sits protected when I carry it around. I do the same with my new 2012 cmbp. I might buy a sleeve, but I won't use a case at any rate.