Pro's:
I'm a control freak, and Gentoo is easier than the other distro's to control (in context of I am allowed the control, not that it is actually 'easier' to control.) It doesn't come pre-packaged with a DE or anything else, not even portage (equiv. of Yum/Yast/apt-get/etc.) so even USE flags etc. are not essential. This is a winner for me. My machine, my choices. On the note of USE flags, they were recently changed. No longer do you set global/local USE flags in /etc/make.conf or at run time, you can now also use individual package names in /etc/portage/package.keywords, along with mask handling in similarly named files, on a package level instead of world.
Everything is compiled to the specifications of MY machine and MY settings. Not someone's dedicated binary maker. If I don't like something about a particular eBuild (source package) that's no problem, I can download it, change it, rebuild it then install.
Gentoo is 'old' as far as distro's go, so it has massive support/user base.
Con's:
Installing can take a while.. after my initial setup I started off compiling KDE. Went to bed, woke up 10hours later and it was still going... I estimated 17hours total compile time for KDE monolithic install. However when I swapped to module install it was significantly less time, but still of epidemic proportions - about 10hours. However normall applications take not much longer than installing via a wizard does.. Eclipse, for example, was downloaded, compiled, installed and running within 5 minutes.
That's about it.. the only other con's I can think of are attributed to the Windows vs. Linux debate. (Lack of support from vendors etc.) and not Gentoo.
I've experienced:
Slackware
SuSE (this didn't last long, admittedly)
Ubuntu
Red Hat (v6, 7 and

Fedora FC1, 2 4 and 5. (skipped 3)
Debian (this is nice, but didn't give me quite enough control)
Mandrake (Mandriva before it turned commercial)
and possibly some others. I've also extensively supported Solaris on both x86 and Sun boxes (Sparc5's, 10's and 20's,) versions 5 and 9. Absolute PITA to install - probably why/because Sun offer engineers to come and install for a fee.
I've got my eye on Archlinux but the support/userbase is still a bit small for my liking, and I'm not quite yet done with compiles instead of binaries.
I never emerge -auD world, there's no point. If it's working, and I'm happy with it, it stays put. If a new feature is added to a particular piece of software that I want, I'll update that on it's own. Other than that, minimal installs are done. I spend maybe 10mins a week compiling stuff, and that's usually primarily nvidia-drivers as they have finally realised it might be a good idea to support linux users, thus they are releasing drivers almost weekly.