It has already passed a full month since I bought the iBook 12". And man, this is one of the best buys I've ever made. Just after I got it, the preinstalled Mac OS X greeted me and let me set up the machine. After that, I spent some time installing the applications I needed such as XCode, Vim.app (yes, I really like Vim's graphical version), the Subversion and Monotone clients, Adium X, Firefox (because Safari fails to open my faculty's website), NeoOffice/J, The Gimp, Colloquy and a bunch of other less-important things. (I haven't had time yet to play with some of the integrated applications.)
Since then, I have been doing real work exclusively. The machine has been rock solid and I haven't had any single problem with the hardware nor the software. Not to mention that the system and the applications have taken care to update themselves whenever needed, so I haven't had to administer the box. I now understand when people say they want to use computers rather than fight with them...
Anyway, the first idea of this post was to summarize the things I like most about the system itself, so let's continue with that. First of all, I'm in love with Exposé. Before I had the Mac, Mac-users often told me how great this feature was, but I was really skeptical and didn't believe them. My mind has changed completely: this feature is truly amazing. Visually selecting the window you want rather than having to look for a name in a task bar or window list is a lot easier. Not to mention that the way it behaves is a lot better than some free clones (which I had tried and didn't really convince me).
Secondly, there is the Dashboard. I find this feature useful because I have tools like a dictionary, the weather report, iTunes control and several other things quickly available after a mouse gesture. Indeed, GNOME provides much of these features in the task bar, but they end up bothering me because they remove a lot of useful space from it. Having these applications in a "secondary screen" is nice — of course, you can achieve something like this with an auxiliary virtual desktop.
Then, there is the concept of active application vs. active window. I also was quite reluctant to have the menu bar at the top of the screen; in fact, I'd already tried this behavior in KDE but didn't convince me (possibly because its applications are not designed to work in such way). I've now come to like it: the menu bar is quickly accessible, as it is placed along a screen boundary, and when the application is active, all its windows come to the front layer. Furthermore, the menu layout is well standarized across applications, so the most common options are available in a known place even if you've not used the program before.
There is also Spotlight, which quickly searches your hard drive for files matching a given search criterion. It is nice, but there is already Google Desktop for Windows boxes or Beagle for Unix ones, so I don't see it as revealing as the other features described here.
Not to mention its BSD core. While the system is designed for desktop usage, having a Unix system underneath is excellent for development tasks. Add pkgsrc to the mix and you can get any Unix software you want painlessly :-)
At last, although I'm also a bit reluctant about visual effects, the ones in Mac OS X are well done and do not get in the way. They even help you in your daily work by, e.g., showing your active window at a "upper" level (using a drop shadow), letting you put some windows in transparent mode so that you can work on two documents at once and other similar things.
Conclusion: I don't regret at all having bought the iBook and I really like its OS, Mac OS X (pity its UI is not free). Of course, I won't leave NetBSD nor pkgsrc development, as I already have a quite long to-do list for them (on which I'll work when I finish this university semester) and I believe in them! ;-)