The new 13" MacBook Air is currently my "middle machine." The Mac Pro is the beast I use at my desk, and my iPad is the barely there waif that travels with me to every meeting and every business trip. The MacBook Air is my hardcore Keynote (presentation), and mobile content creation machine. I say hardcore for a reason–even long form messaging and writing is pleasant on the iPad when a keyboard is used.
Anyway, here's a very nice review from someone who uses the MacBook Air as their primary machine. If a had a terabyte less of data, I'd love to use the MacBook Air as my one-and-only as well.