I've been using FreeBSD for years now (everything from my first 486 apache web server w/ 16mb of RAM to a compaq proliant quad processor server w/ 4gb of RAM) and 6.1 definetely doesn't let the product line down. I wouldn't run anything mission critical on any other OS.
LAN Party server
There are also a ton of great howto articles written for completing certain tasks. While its shortfalls might be in the desktop area, you can't really compare a Server OS to a Desktop OS can you. If anyone ever wants advice on building an infrastructure around a Solid OS, I can't imagine why it wouldn't be this one.