I had a Dell laptop before it died (after just 2 years), and I decided to go for greater mobility in my next laptop purchase. The Dell was 10+ lbs. with case, power adapter, and all. The powerbook weighs half that (the power adapter weighs next to nothing and the iBook is rugged enough to just toss in my backpack. 5 lbs. versus 10 lbs. might not seem like much of a difference, but trust me, your shoulders feel the difference.
For an ultramobile laptop, you just cant beat the value of the 12.1" iBook. I went for the $1300 800MHz model because of the combo drive - it makes a great portable DVD player and it's great to have the ability to burn CD's. I looked at ultramobile models from Sony, Dell, and Fujitsu, and their models with comparable features cost at least $1500, and in my opinion the iBook still had the better and more complete feature set.
I really, really like OS X. The iBook has is the most legible 12.1" screen that I've seen, what with all the font smoothing in OS X and the iBook's very crisp display. And the iApps are awesome. iTunes is so great - it's the only MP3 player that I've been totally satisfied with (having tried probably at least a dozen PC versions). And if you have a digital camera or video camera, iPhoto and iMovie are also great.
Get the 12.1" Combo drive iBook for $1300 from a reseller, they'll throw in an extra 256MB of RAM. Or get the $1000 iBook if all you need is a CD-ROM. Believe me, you'll be very satisfied with your purchase. I bought my new iBook almost 2 months ago now and I still think it's the best computer purchase I've ever made.