I use my Macbook air (13") as a primary PC. Yes the HD is small, but you can also back up onto a portable HD, and you can buy an external DVD/CD player (not that I ever have the need for one).
Pros - awesome battery life. I can get 5-6 hours or more, depending on what screen brightness I use and what apps I'm running. It's also easy to use and super portable (a must for me). The solid state HD also makes things like start up and shut-down incredibly fast. The design is also very sturdy, and has already withstood a 3 foot drop onto hardwood, resulting in no damage. The screen is bright and super clear, too - something that I am quite picky about, as poor video quality is a pet peeve of mine.
Cons - Keyboard feels a bit cheap. It would have been nice if the keys were larger or if they were a bit more raised - I immediately felt it was harder to type as rapid/accurately as my old computer. It can multitask, but it WILL hang up if you try to run too many apps (especially processor intensive apps) at the same time. I don't play any computer games or do any serious video editing/processing, so I can't report on the overall performance accurately.It also can overheat if you are in a warm room or if the air vents are covered. No HDMI or AVI output - you have to run it through the Mac port, requiring you to get an adapter and making audio through HDMI essentially impossible. Also, I definitely prefer any Windows (XP, Windows 7, etc.) over Apple's OS. OS X is easy to use but at times it feels a lot more tedious - I really miss all the little shortcuts on a PC that helped me be more efficient (even simple things, like page up/page down).
Hope this helps! Good luck with the choice.