All the BSDs have Linux binary emulation built into the kernel (they just need some libraries adding). It's also pretty simple to modify Linux source code to run on a BSD box natively. The only real differences are the kernel, the kind of file system used, and the way devices are handled. They all run a lot of the same software, like KDE/Gnome, GIMP, so to the average Joe look the same, when compared to Windows or OSX.
I've only really used OpenBSD (and OSX).
NetBSD runs on almost anything, but doesn't seem to be maintained well enough to make it useful on any of the stranger hardware (My Jornada 720 has JLime Linux instead of the hpcarm port of NetBSD, because JLime is actually usable without compiling everything from source, all without any docs, or even platform specific source code).
FreeBSD seems to have been going downhill, and seems to carry too much old code to be updated properly, or run as well as it could (I ended up using Gentoo on my Dell PowerEdge 1950 game server, because FreeBSD hasn't been updated to include 64bit Linux binary emulation, despite 64 bit being the norm for years, and the FreeBSD side being 64bit. The BF2 server package is 64bit unless you're happy running an older buggy version).
DragonFlyBSD looks to be something with amazing potential, but it's a still pretty new, so doesn't support as much hardware/software as the others. It's being developed as a Virtual Machine hosting platform, and has a lot of very up to date programming ideas going into it to maximise performance.
OpenBSD is the one I've been using constantly for 6 years on my firewall/gateway/router/load balancer/web server (and it got me using OSX instead of Vista, because underneath it's BSD). It's designed from the ground up to be as secure as possible, so it's very well coded (because doing the regular security audits on the whole codebase would be a lot harder otherwise), and easy to use in some respects because if it wasn't you'd take shortcuts that would reduce the security. It's not really a desktop system, although you can get all the usual stuff for it. It's more tailored to be run on a firewall/web/mail server, and has a lot of tricks for that purpose that won't be found outside very expensive hardware from companies like Cisco.
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