I like OpenBSD for its "solid" kind of feeling it gives me right from the start (installation, that is). I LOVE the default fvwm configuration you find yourself with after the base install -- actually I've borrowed that config as a basis for my own one.
GPT support is there now, but one needs to be careful during the installation while installing on anything else than a full-disk GPT partition. Finally I've figured out the way to install OpenBSD on a GPT partition without having it overwrite the whole disk GPT table (in case you're installing on a GPT multiple-partitioned disk). And still, it will only agree to install on a GPT partition that's 2nd and is preceded by a EFI system partition.
The next thing I like is the /etc/hostname.if NIC configuration feature. And PF, of course.
But I have this problem with the KMS X driver for Intel Graphics (on my Lenovo T61): once I log out of the X session the screen goes considerable darker and remains so. Does that happen only on some hardware, or is this the way the driver behaves in general? Currently I'm on OBSD 6.0. FreeBSD has solved this problem quite some time ago and you can log in and out of X as many times as you wish without your screen visibility being affected.
I've been able, following an on-line tutorial, to make a USB pen-drive based OBSD system (all mounted read-only + tmpfs for writing) containing all the software one would need for a gateway|firewall|whatever machine. Not possible with FreeBSD because it needs a swap partition.
...And lastly, OpenBSD precompiled packages seem to be much better than those for FreeBSD. At least, a couple of years ago I wasn't able to use some precompiled packages on FreeBSD and since then I've always built stuff from source. Not so on OpenBSD.