I am using the Midnight Blue Apple leather sleeve. It works really nice when you just want to have the Mac with you when you go somewhere. I was looking for cheaper options but none of them are as sleek and lightweight. Also other leather sleeves I was looking at had some tacky logo or company name on them. The apple one does has an apple logo but it is subtle. I also have a Waterfield leather messenger bag if I want to take more stuff other than the Mac, but that has a padded compartment so I don't use the sleeve with it.
I have a InCase tensorlite sleeve for my 12" MacBook and it's a fantastic choice if you want something with a little more padding but is also slim. It comes in all the sizes.
InCase Icon Sleeve