I think a hard shell case like Speck's is a must if you are going to slot the Macbook Air in with a bunch of other things in a bag.
Other than that, I have found that the screen quality is pretty good, and applying an extra layer of protection may either make it darker, which would kill sunlight legibility, or cause scratches and other problems during application.
Personally, I love my Macbook Air. With only 2GB of RAM, it doesn't feel limiting in day to day use. I mostly use it to type up documents, compile data in spreadsheets, prepare presentation, make adjustments to photos in Lightroom 3, work interface graphics in Photoshop CS5, and run Windows via a VM whenever I need access to certain applications.
It has worked very well so far, much better than other laptops I have tried over the years. The screen is especially good as it is very legible under direct sunlight despite the glossy layer on top. But I like glossy. It's easier to clean and wipe than matte.
Overall, portability, screen quality, snappy performance, and solid build quality is what I love about the Macbook Air. It has lightened my load during day to day commute by a lot. My Macbook Pro was much heavier, and now that I have the Air, I can't fathom using a heavier laptop. This has got to be the best purchase I have made in a long while.