I do not think there is any real benefits with todays CPU's. I left because it became a nightmare to maintain with the overlays.
Arch, for me, has struck a balance between power and usability, I can never feel at home with any other distro.
Compiling everything from scratch, for over 3 hours is going to gain you how much of a performance boost? Negligible IMHO.
* Gentoo offered a package manager similar to ports on BSD when there was none in Linux. In the olden days you had to hunt down the developers site to find the software update you were looking for, hours a day wasted on Google. Nowadays every single distro has a well maintained packaging system, so not needed for that anymore.
* Gentoo offered bleeding edge software and updates. In the old days distros only did security updates and only upgraded software on a new distro version release. If you wanted an update of software for a specific reason, see above.
This propelled Linux app development into the future, developers now had a reason to release regular updates of their apps. Nowadays all distros offer bleeding edge with either "testing", "unstable", etc. repos to try out.
* Gentoo gave you an escape from dependency hell with their package manager being able to locate and satisfy all needed dependant libs, etc, something that was implemented in other distros but not nearly as polished as emerge. Emerge was light years ahead of the rest.
This led to system maintenance being a breeze in the beginning. Now every single distro offers this.
This is all I could think of quickly. Gentoo did it's part for Linux in early 2000-2004, and there is no doubt in my mind that it played a vital part in getting Linux distros organized and maintained within set standards, but with the powerhouse CPU's of today and distros having adopted everything Gentoo once stood for, I see no need other than curiosity or for learning more of Linux that one would try Gentoo.
That said, everyone ever wanting to create their own distro should first get to grips with Gentoo or LFS to learn how to optimize a distro before just spinning off another *buntu clone.