I bought my MacBook two years ago, and at first I was cynical about how I would find it compared to Ubuntu, but over time I found myself using it more and more. While I have never considered myself a Mac user (I've always thought of myself as a Linux user who owns a Mac), I did grow to appreciate the combination of a solid Unix base with a well thought-out GUI, and it worked out well for me.
Yet now I'm finding myself slowly moving back to Linux. I'm running Slackware 13.1 on an old Inspiron I've had since 2004, and my recently purchased Dell Studio 15 dual-boots Windows 7 and Ubuntu. But the one I'm using by far the most is my Mini Inspiron, which dual-boots Windows XP and Ubuntu Netbook Edition, and XP almost never gets used.
Part of the reason is that the Mini itself is just such a handy machine to have around - it's small enough to be portable, and it's convenient to use it anywhere at home too, but at the same time it's big enough to use without any issues (which is more than I can say about my old Eee PC, which has now been passed on to my sister). OK, not everyone likes Dell's products, but I've always found them to be very good value basic computers, and they really have gotten the whole netbook idea and produced an excellent product.
Also, Ubuntu Netbook Edition is really great - the newly revamped user interface works really well on the netbook form factor, and it's an excellent experience for the most part. Performance is more than acceptable, and most applications start very quickly (Chrome is noticeably faster than on OS X).
But the thing that I'm really starting to appreciate more than anything else is just how much more powerful and flexible it is under the bonnet than OS X. For starters, a package manager like apt-get makes it incredibly easy to install new software. In particular, this is useful for installing useful Python modules, as so far as I'm aware Python doesn't have anything like Perl's CPAN or Ruby's rubygem system. Yes, I guess I could use MacPorts or something like that, but personally I've never liked the idea of installing a new package manager yourself rather than using one that's a fundamental part of the OS.
This also means I have to compile many Python modules from source on OS X. This has taught me an interesting lesson about usability - namely that it's not an absolute. For many tasks Ubuntu is actually considerably easier to use than OS X, since Synaptic offers users the opportunity to install a lot of software from a GUI that on OS X would need to be compiled from source, although for installation of desktop applications OS X is probably marginally easier.
Also, byobu is an excellent tool - it really helps to make GNU screen more useful and accessible. I have it set up to launch screen in any new terminal sessions and I'm beginning to wonder how I ever coped without it.
Finally, Snow Leopard seems to have broken a lot of things I liked in OS X. I used to use the Terminator terminal emulator in Leopard and it worked well, but in Snow Leopard, Spotlight can't find it (I think it's something to do with the Java bridge being deprecated, as Terminator is written in Java), so I had to change to iTerm instead, which I don't like so much. Snow Leopard also seems to refuse to open browser bookmarks via Spotlight in anything but Safari, which really gets on my nerves as I hate Safari.
I'm not planning to get rid of my MacBook anytime soon, but I am increasingly finding it's less appropriate to my needs, less productive, and less powerful compared to Ubuntu.