I'm running both systems at the moment (had gentoo since 3 years or so from 1.4 rc and freebsd 5.3 since a few weeks). I use my pc as a desktop environment for most every-day tasks, programming, watching movies/dvd's, playing games (ID soft mostly) etc...
Frankly freeBSD beats the living daylights out of gentoo. KDE feels so much snappier on freebsd and I didn't have to tweak any silly cflag, nptl or prelinking to do this, it just was like this with the standard settings. Same goes for any other application in there. I get better framerates on games too (enemy territory, doom3), which is weird considering my nvidia geforce3 runs linux glx drivers on fbsd and the games are also static linux versions.
Upgrading is very painless as well (granted, only had it for a few weeks but upgraded kde and x.org already in that time), bsd-STABLE has packages that are generally a version ahead of portage's stable and the system has only been solid as the mount everest.
I don't have any bad experience with gentoo, it works well, fast, portage owns etc. but after this short while with freebsd I must say that managing the useflags and what packages I want (like quicktime on mozilla) to work takes some effort which isn't the case on freeBSD, things just work the way they are supposed to. Using make files instead of emerge also means you can turn the pc off halfway the openoffice compile for instance, boot up in the morning and make will just continue where it left off (emerge --resume starts from the beginning of the build it was doing), nifty if your pc is near your bed.
I recently installed FreeBSD and then uninstalled it because it did not have drivers for my nForce2 Ethernet card, or at least not any that did not involve lots of screwing around. What I used of it was nice though.
P.S. - If anyone knows where I can get some nForce2 Ethernet drivers for FreeBSD please point it out so that I can use it.
My system is an epox 8rda3+ (nforce2 ultra) and a simple google revealed exact instructions on installing a linux nvnet driver on freebsd which hasn't failed since it got installed. Apparently freebsd can even do windows nic drivers but I haven't used this feature in any way.
This brings me to the documentation. If you read some instruction on how to get something done for freebsd it will be correct to the last character because of it's cohesive nature, which isn't the case on linux (these gentoo forums help, but still isn't as consistent as fbsd).
Then maybe the 1 thing I found isn't up to scratch on freebsd is the sound on some programs. Owning an sblive! I get really good support on all platforms, but the sound on xmame is just totally terribly choppy, dunno if it's xmame's fault, the port or whatever. Turning the eq on xmms has a small delay (like windows' winamp) but apart from those things, sound is fine. Quake3, doom3 etc. all have sound working with no delay at all.
In short I like both systems, but freebsd seems to be my favorite for nearly everything I do with it cause it just works better. The linux-compatibility works as just a thin wrapper around the bsd kernel so there is no slowdown at all, on the contrary. This is very handy if you don't want to compile openoffice but just use the binary linux version.
I'll put my system specs on here just for the comparison value of the 'rant'.
Athlon-xp 2500+
512MB DDR 333MHz
GF3ti200
FreeBSD 5.3 with updated ports (meaning kde 3.3.2, x.org 6.8.1)
Gentoo on 2.6.10, nptl, prelinked, march=athlon-xp -O2 -fomit-frame-pointer -pipe -s, kde 3.3.2, x.org 6.8.0-r3 (isn't this 6.8.1 as well ?)