I commute to work on a bus daily, and travel some, so to me, protection is #1. I want to be able to throw my 600 in my bag and not have to worry that it is perfectly sandwiched between things to protect the screen. I really don't like that the Sony cover is so soft. So, I spent big to get what I feel is the best setup for protection and usability.
I got the Sony Standard cover (I've already written about how I feel about the cover with light) and a BookArmor cover edition (found here
http://www.ecaddycase.com/coveredition.html). I got the Sony cover for $17.99 with a 40% off Borders coupon, and the BookArmor is $35. So yes, about $60 (with shipping and all) for this.
The BookArmor is a form fitting zipper case with plastic and metal inserts to keep the screen safe. They have a version that you clip your unit into "naked", but it looks pretty janky. The cover edition holds your reader in place using the Sony cover. So, when I'm traveling to work, I zip everything up and it stays nice and safe. When I get to work, I pull the reader out of the BookArmor case, but leave it in the Sony case so it doesn't get scuffed up laying around. That way it stays protected without the slightly bulkier case. I've been using this setup for about a week now, and it's great. The bonus is that the case gives me something to clip my reading light to on the bus.