I never saw the need. My backpack is a 25 liter top-loading duffle with no laptop compartment. My 15" MBP, books, clothes and lunch(es) go in before I bike to work, repack before heading off to class, and everything manages to survive the 40+ mile round trip days just fine. I usually pack the clothes closest to my back, then sandwich the MBP between some bigger books (Calculus and Chemistry, for example) with the lunch(es) on top. If it looks like rain, I squeeze in a regular kitchen trash bag over the top of all of it and an extra one folded up underneath. All together, the silly thing weighs 25-40 pounds depending on the weather and the classes. Seriously considering some nice weather-proof panniers come winter. Anywho...
The MBP probably has a few scuffs along the back edge that it wouldn't have otherwise. But it's held up just fine, even survived a 20+ mph crash better than I did... Seriously, a nylon sleeve will be plenty to keep it looking pretty. As long as you aren't slamming your backpack into walls or tossing it down the stairs, the computer can handle the "abuse" of daily life.
The only case I've ever actually considered was the Speck hardshell. They come in a variety of attractive semi-transparent colors, fit well (according to the reviews - be sure to buy the right one for your release date!), are built to last (according to the reviews), and very reasonably priced (even at the Apple store).