As a Mac user that runs triple boot, the "just works" mentality in OS X is present only as long as you don't step out of the typical use case scenarios. It's great to say be a writer on OS X; there are very good tools and apps to write in a minimal, esthetic environment (WriteRoom and others). On the other hand, as a "hacker", I've always had trouble in OS X -- from Page Up/Page Down not registering properly in Terminal, to full NTFS and EXT4 support (FUSE used to work, but I think it was not maintained) to getting vim, gcc and other tools of the trade working on OS X (yes, there are ports-like systems on OS X, but hardly ones that I'd say "just work").
On the other hand, in pure Linux world, brightness controls, backlighting controls, volume controls and multiple sound card support were all rather tricky when I tried using Gentoo on the same Mac for a time. Some of those I solved, but some I just gave up and was content that it kinda-but-not-completely works.
Ubuntu seems to be tested enough on Macs, so those things work on my laptop now without having to configure too much. It may not be the perfect "just works" experience, and I have seen a lot of things going the wrong way on Ubuntu, but currently it's a good mix of being easy to maintain and being easy to start doing actual work with.