OpenBSD is the way to go.
Running it on my server and my laptops.

I run only free software on my desktop, and I am using a Dell Dimension 4100. I have used FreeBSD in the past. It works fine, but compared to the Linux distros I use, it can be a challenge to install. On the contrary, PC-BSD slips right in, I can use FreeBSD ports, so i have everything I need. Therefore, for desktop use, PC-BSD, definitely. I have also used DesktopBSD and FreeSBIE. PC-BSD is tracking my needs a bit better at the moment. All of 'em are good, PC-BSD just suits my needs a bit better right now.

Most of the time I run desktop Debian GNU/Linux software - my latest version is sidux, a variation that smooths out the bumps in Debian Sid and provides bleeding edge software that works great for me.

From the BSDs, OBSD suits me best.
I run it on an old athlon.

When I had a bsd, other than a Mac OS X, I ran FreeBSD, which I find is very user friendly.

[Database/webserver/firewall] FreeBSD 6.2/i386
Dell Precision 410
2 x 128MB = 256MB - mem
2 x Pentium 400MHz
2 x 18GB HD
1 x Via, 1 x 3Com, 1 x Netgear NICs

[External Snort Server] NetBSD 3.0.1/i386
Dell Precision 220
128MB - mem
Pentium 800MHz
10GB HD
3 x 3Com (two bridged and connected to an ethernet tap, one for mgt interface)

[Internal Snort Server] OpenBSD 3.8/i386
Homebuilt (Pine motherboard)
Pentium 200MHz
4.3GB HD
196MB ram - mem
1 x Via, 1 x 3Com
S3 Virge-PCI 8MB

[Workstation] Mac OS X v10.4.11
PowerMac G4
2 x 533MHz
60GB HD
1 GB RAM - mem

[Notebook] Mac OS X v10.4.11
Macbook 13.3" glossy
Pentium 2GHz CoreDuo
768 MB - mem
80 GB HD

I've several other *nix machines:

1 laptop dedicated to Slackware
1 laptop dual-booting XP and Slackware
1 1U to replace that will run 3 Snort sensors on different net segments, running Slackware

Well I haven't got a lot of BSD experience (read that none!) been a Slackware guy for many years now but decided to take the BSD plunge. Dove right in and installed OpenBSD on a spare HD. I'll be playing and learning. Should be fun!

I switched from Slackware too. I couldn't believe how 'easy' OpenBSD was compared to Linux. Everything just works... or won't work at all :)

Nice. There's something to be said for decisive victories and failures.

My favorite is FreeBSD, but I haven't ever tried Open, and I've only dabbled a little in Net. So take that for what it's worth. It is very difficult to leave a fantastic OS, and I feel like there is always something new to learn with Free.

I installed v1.4 of PC-BSD on a free partition of my laptop recently, and I was impressed how smooth the install was. That being said, I prefer Gnome to KDE, so I am sticking with Ubuntu for now on my laptop. Aside from that I run FreeBSD on a home gateway/firewall/server/WAP, and it has been a great experience.