Alright, got it installed, and it boots just fine :) (Had an issue with my custom kernel, where a special config needed by systemd wasn't activated).

As far as I can tell, here are my first impressions:

It's extremely easy to install, and relatively easy to configure. The package manager, paludis, has a nice interface called "cave", which will help you manage your packages. It seems extremely powerful (it's inspired from portage), but also really different from what you are used to when it comes to package management.

For example, you don't "install" a package, but instead, you ask paludis to resolve an issue for you, where your issue is to have a package installed (seems pretty weird at first, but makes sense after all).

The difference between "world" packages (explicitely installed) and dependencies is also nicely drawn. Thus, dependencies seems well handled.

Though the package management systems seems to be targeted to people that already know how `portage` works, and are rather used to it, as I couldn't find any simple tutorial about it. There are still things I don't understand (eg, I can search for packages, I get results, but when trying to install them, I just can't because of the "mask". Not sure what it is yet).

So in the end, exherbo seems like a good improvement over gentoo, with an extremely powerful package manager.
I totally recommend trying it !