Hello there.
First of all I have to say that this is a biased opinion because I use Gentoo for my home desktop and server machines.
However, I have used Red Hat, Mandrake, Suse and several Debian based distributions.
I'm now running Gentoo only and I have never used Slackware on my home machines, so I can't give advantages/disadvantages over that distribution, all I do here is explain some good points of Gentoo.
Sorry but this is quite a long post, and I know I'm a zealot, but I hope it is helpful.
The points I would like to make about Gentoo is that It's not just about performance gains by compiling everything with your specific CFLAGS.
OK some people may argue about whether this actually increases performance or not, personally I think it does because a lot of distributions use generic optimisations -march=i686 or something, where as I compile packages on my Athlon with -march=athlon-xp .
Choose for yourself if this makes a difference, I'm not here to argue about that, neither have I hard facts or proof about whether it is true. It is just my opinion.
You do not have to compile everything when you install Gentoo, if you are comfortable building a kernel, then I think you will have no trouble, this is the only thing that needs to be compiled when installing, although you can compile everything if you wish.
The Gentoo Reference Platform (GRP) has already compiled packages that you can emerge that are built for your specific CPU, you can install the Gentoo base system, KDE, GNOME, Xorg and lots of other software from these binary packages to save time, these packages are only for installation though, and their USE flags are preset.
An option which I haven't checked out is
http://chinstrap.alternating.net/ which provides current binaries to save you compiling updates.
Maybe I should say that I run Gentoo on a 150MHz Pentium and I compile everything, this machine is always on and I set portage to compile at nice 10 and I hardly notice it when I'm updating and logged in through ssh.
By far I think where Gentoo shines the most is the USE flags, maintaining updates with dispatch-conf and when you want to mix stable, testing, and unstable software.
Suppose you never want packages with GNOME support built in, you could put USE="-gnome" in /etc/make.conf and no packages will be built with that extra baggage.
If you want a specific package with GNOME support, you could do that too while still keeping your general policy of no GNOME, this is a really a bad example but I hope you get the idea.
When you want keep your stable system but you want to check out the latest GNOME releases (beta) it is simple to just unmask GNOME without having to install a ton of other unstable software just to get it.
It Is also very easy to create your own ebuilds for portage and the Gentoo community and forums are very friendly and helpful if you get stuck.
Here are some Gentoo related links.
USE flags
http://www.gentoo.org/doc/en/handboo...ok_part2_chap2
Mixing Software Branches
http://www.gentoo.org/doc/en/handboo...ok_part3_chap3
Additional Portage Tools (dispatch-conf)
http://www.gentoo.org/doc/en/handboo...ok_part3_chap4
The only way you can find out if you will like it is give it a spin, everyone has different ideas about distributions. Me - I like Gentoo
Take care and best regards