I tried running 7.0-RELEASE for a while, but I went to 8.0-CURRENT and never went back. The iwn(4) driver on 8.0-CURRENT works a lot better than the "unsupported" 7.x driver mentioned in another thread.
The thing to keep in mind is that ACPI support is only so-so. Volume controls, screen brightness and other Fn keys don't work "out-of-the-box", and it will only resume from suspend if you're running Xorg and using only one CPU core.
I have to say that NetBSD 5 (Beta) works nicely, and of course recent Linux distros work well (Ubuntu, Fedora, and Gentoo).
I guess if you're a hardcore hobbyist like me it may be fun for you to run FreeBSD on the X300. I'm going to try some suspend/resume patches I saw floating around on freebsd-acpi mailing list and see if I can get my other CPU core back, but other than that, I look forward to tracking 8.0-CURRENT and seeing all the nifty things the FreeBSD devs are up to.
Good luck!