I recently moved my main machine over to Arch and it is a good distro.
As tln says, they are based on binaries, the tar.gz or .tgz files names are simply because the binaries are tarred then bzipped to make downloads smaller.
However, you can install anything you like with the source code as Arch also has something called abs - a repository of, for a comparison, 'ebuilds' that allow installation via source code (or you could do it the old fashioned way - by hand).
You can make abs files for others to use (like Gentoo users sharing ebuilds).
It is slower than Gentoo (though not a lot) but just as easy to use and the forums, although much, much smaller than these ones are just as friendly (in my experience anyway).
Also, as Mystilleef says, the documentation is pretty bad at Arch but if you use Gentoo then you won't really need to use them - I only glanced at one and that was the alsa config for Arch, everything else is pretty self-explanatory and the config files are described adequately in the install manual.
The only other advantage over Gentoo that I can think of is a full working install in about 40 minutes. I like it, but will keep my laptop on Gentoo as it is optimised for my old pIII
