BSDs are mostly used as servers. OpenBSD is very security-oriented and makes an excellent firewall. FreeBSD has very good network performance, it's ideal for file servers. NetBSD is ... well, it's your basic free Unix-like operating system and it runs on many hardware platforms.
BSDs can also be used as desktop/workstations. In this case, FreeBSD is perhaps your main candidate. FreeBSD 5.2.1 has more flexible installer, more detailed documentation (the FreeBSD Handbook), and support for newer hardware than the other BSDs. There's also a FreeBSD based LiveCD called 'FreeSBIE' ( http://www.freesbie.org/
) that allows you to take a look at what FreeBSD feels like on the desktop, without the need to install it to your hard disk first.
Personally, I prefer NetBSD because it's so ... well, it's just so BASIC. But I'm using NetBSD mainly to learn more about Unix and people who have different interests may prefer some other flavour of BSD.
@chort: 'the nightmare of a Debian install' -- I agree with you there
but you should know that the new Sarge Debian-installer is currently at the state of beta testing, and it will be a bit easier.