As for me, I don't see any real use for Gentoo. It's a kind of toy OS for those who just grown older and thinks Lego isn't cool enough for them. It's basically not targeted on any serious use but rather to those who want to have a playground and/or system lab. Some guys sometimes can construct something impressive from both Lego and Gentoo, but this does not implies than building machinery from Lego or servers from Gentoo is a normal way of doing things.
You see, I don't need Lego-like OS on servers, so I have to build or fix half of things myself, fixing stupidities here and there.
Gentoo has two interesting properties that most other linux distributions do not have:
a continuous update path, no need for a full reinstall ever
minimal (and often no) patching of upstream sources, making solving problems usually easier
It looks like that arch is there too, if you look for the same characteristics in a binary distribution. Not sure about their patching level though.
And the number of stupidities to fix is rather low nowadays. A little higher that for a binary distribution due to the somewhat extreme configurability, but not bad. Running gentoo is nowhere near as "l33t" as it was some years ago.