Last year I installed FreeBSD 4.9 on my laptop as part of a dual-boot setup (with Windows XP Home on the other half). Here are some thoughts, keeping in mind that I haven't played with it too heavily:
Fast - I've run various I/O benchmarks of my own on Solaris, FreeBSD, Linux, HP-UX and Windows. FreeBSD always seems to get the best performance, and the filesystem just *feels* fast in general.
Secure - more so than Windows and even Linux (based on Security Focus and user comments in general)
Installation - Ugh. If a FreeBSD expert hadn't been sitting *right next to me* the whole time, I never would have gotten this thing installed.
Unable to write to NTFS - Yep, it's read only. Pain in the arse.
Despite RTFM, I can't get my (definitely supported) wireless card to work.
Ports - Feels like a weird halfway point between building from source and, say, Solaris packages. There do appear to be packages, too, and I've used those when I can. Maybe I just need to get used to it.