Nick, a sleeve can be used alone, if it is like the Waterfield sleeve where optional D-rings and a removable strap are available. Less to carry than a larger bag, but little or no space for other items like a Coolpad or mouse.

Or the sleeve can be used when the portable Mac is carried in a courier bag, where it adds additional protection, particularly if the larger bag doesn't have sufficient padding.

Lou, you don't like the Waterfield messenger bags aka Courier bags? They are designed to be used with a SleeveCase.

Here is a well-written review that includes a lot of exterior and interior photos of the Courier and SleeveCase separately and together:

http://www.the-gadgeteer.com/waterf...rgo-review.html