The 15-inch is the perfect size for me. I looked at both the 12 and 17-inch models and felt they were a little too small/big respectively. There is something attractive about the 12 inch - a friend of mine has one and he carries it around everywhere, able to write anywhere anytime. And seated at a desk the 17-inch is also attractive, presenting a massive, bright screen to work on. But all in all, the 15-inch provides the best both worlds. It's very portable - maybe not as easy to tug along as the 12-inch, but certainly manageable with a small backpack. The 17-inch can be problematic on a plane if there are people crowding you, and it's certainly cumbersome to maneuver in tight spaces, whereas the 12-inch is easy to maneuver but can cause a little eyestrain from a distance.
I definitely recommend a Mac, first of all, and within that I recommend the 15-inch G4 notebook. If you must have a 12-inch, you might take a look at the 12-inch iBook, which is cheaper but obviously not as feature rich (the iBook's graphics card is not as powerful and the processor is slower, most notably).
The complaints I have are exterior - the computer doesn't age well (i.e., it nicks and scratches rather easily) and there should be some sort of elevation device to raise the back end while typing (like the older Powerbooks had).
All in all you can't go wrong with any Apple laptop you choose. They are truly making the digital life a reality with all of their integrated software - iTunes, iPhoto, iSight, and the iPod are particularly cool. No other laptop will have an operating system with so many fantastic programs that are so perfectly integrated together. If this is remotely in your price range, go for it. It can literally change the way you operate on a day-to-day basis.
