I'm a professional UNIX admin. I've worked extensively with both FreeBSD and Solaris for years. Most of my recent work experience has been with Solaris 10, but I've run FreeBSD at home for years.
I recently needed to stand up a new application server at home. I considered using Linux, using OpenSolaris, or using FreeBSD.
I considered Fedora because the handwriting is on the wall where I work: the company will not permit new Solaris installations, in large part because it's not clear that Sun will still be a viable concern in a year or two. The corporate direction is to move to Red Hat. However, I quickly became infuriated with the poor quality of Fedora's documentation. I couldn't find clear answers to setup questions. This wasn't a problem with either FreeBSD or OpenSolaris. This took Fedora out of the running for me.
I decided to try OpenSolaris, because I know Solaris 10 and it might be useful to have the extra practice system at home. But OpenSolaris isn't Solaris 10. It doesn't have the driver support.
What really caused me to wipe out my OpenSolaris install and go with FreeBSD, however, was learning that Sun doesn't even supply security patches for OpenSolaris. If a security issue arises, you either have to wait for the next OpenSolaris release, or go about rebuilding from source. If you want prompt security patches, you have to pay for a Sun support contract -- and pay just as much as you'd pay for the "commercially supported" Solaris 10.
This astounded me. On Solaris 10, Sun provides critical security patches free of charge. Why does the "commerical" package provide free security patches, but the "open source" package doesn't?
There are features in OpenSolaris and Solaris 10 that FreeBSD doesn't have. But, speaking as a certified Solaris admin, I have to say that FreeBSD is more supportable if you can't afford the Sun support contract.
So, I would, and did, go with FreeBSD. It works great, it's solid, it's well supported, it runs well on all sorts of hardware, and it's likely to be around for a while. If the European Union drags out the Oracle/Sun deal much longer, I don't know that Sun will be able to avoid liquidation. Even if the deal goes through, Sun has a big challenge; a lot of their best customers have pulled away because of the uncertainty -- and the decline in support quality over the past year or two. I don't think that Solaris experience means quite as much as it used to on a resume.