FreeBSD is a great operating system for servers. I had an Intel Celeron server with 512mb of RAM on it and it ran lightning-fast with FreeBSD, even with a high-traffic site.
I am not sure about using FreeBSD as an OS for a PC, as it is more of a server solution. However, there is an OS for desktops called PC-BSD, which is based off of FreeBSD exclusively for PC. I'd love to try this and I have heard only good things...they have a 64-bit version of PC-BSD which I'd love to try.
Unix vs. Linux, I would love to compare the two if I had enough computers, especially with the new 64-bit versions. Bah.