I briefly experimented with it ages ago. And I mean ages ago, like 20+ years ago. Maybe it's changed somewhat since then, but my understanding is that Gentoo doesn't provide binary packages. Everything gets compiled from source using exactly the options you want and compiled exactly for your hardware. That's great and all but it has two big downsides:

• Most users don't need or even want to specify every compile option. The number of compile options to wade through for some packages (e.g. the kernel) is incredibly long, and many won't be applicable to your particular setup.
• The benefits of compiling specifically for your system are likely questionable, and the amount of time it takes to compile can be long depending on your hardware. Bear in mind I was compiling on a Pentium 2 at the time, so this may be a lot less relevant to modern systems. I think it took me something like 12 hours to do the first-time compile when I installed Gentoo, and then some mistake I made in the configuration made me want to reinstall and I just wasn't willing to sit through that again.