I strongly dislike Gentoo Linux, and characterise it as a system mostly useful for people who either come to Linux with the wrong attitude or for people who really must be on the bleeding edge for some reason. Gentoo running on a server or an important workstation (e.g. someone who does not consider Gentoo to be a hobby of its own) is, I think, a mistake. However, I am very impressed at a derivative distribution of Gentoo called System Rescue CD, which does an amazing job at being a swiss army knife OSImage for a sysadmin. It's the best equivalent of a DOS floppy packed with all the debugging tools you might possibly want. This is where the gentoo style of thought happens to coincide with something useful, and some credit to Gentoo is due.
This is quite different than Fedora Live (which I have on a USBstick), which is more of a "bootable linux image that's like a desktop no matter what system it's actually running on" thing.
Different yet is stress Linux, which I have been using for burn-in of new systems for quite some time.