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I have managed a farm of ~500 servers running Gentoo and when I first joined the company owning those servers I first thought it would have been a nightmare. Whoever had picked Gentoo had done it for the wrong reasons, packages were all over the place, configuration was scattered, etc... I had used Gentoo previously on a couple of desktops which helped me a lot, but I had never considered running it on servers before.
One year later after much wrangling and fighting we had synched all boxes to the same portage tree and would run updates and configuration changes in lock-step using cfengine, plus custom tuning and tweaking for the boxes that needed special software. Once we reached that point administering Gentoo felt really like a breeze, I was surprised myself of how well it run and how easy it was to manage (and upgrade!).
I don't work there anymore but I'm told by some friends in their IT team that they're going strong and the machines are humming just fine; so long story short, Gentoo is actually quite a good distro for server-side installations but you need some discipline in your IT team to run it properly.
That surprises and impresses me
I used to run Gentoo on various machines at home, and it gave me some pretty big headaches at times. I guess it's all down to a proper maintenance schedule, as I always had my biggest issues after leaving the systems alone for extended periods.