I've used FreeBSD or OpenBSD on my laptops for years but I've recently given up; while they're both nice operating systems (including on the desktop, especially if you use DesktopBSD) they lack some things I need (mostly embedded software development tools and easy support for things such as flash video).
So I'm now a Kubuntu user (it's like Ubuntu but with the K Desktop Environment instead of Gnome). I like the way all KDE applications interoperate with each other (well, this works if you use KDE applications; fortunately there are a lot of great KDE applications including a full office suite, an IDE, the wonderful Amarok audio player, etc.) It may not be as good as Mac OS X but I think it's a decent alternative. And KDE is very configurable and overall well designed.
Now, the bad thing about Kubuntu is that the out-of-the-box experience could be better. You actually have to install support for proprietary codecs and things like that yourself (but it's easy).
A final thing: I've experienced the FreeBSD ports system, the NetBSD pkgsrc system, the OpenBSD ports system and, while they're all good systems, I have to say that the apt system shipped with (k)Ubuntu is by far the best. It just works.