"Using/installing Ubuntu is like buying a car. It may have a few features you'll never need or use, and might need to have a couple features added as aftermarket parts.
Using/installing Gentoo is like buying a pile of sheet metal, a few rubber trees, small pile of copper, a pile of sand, and an oil well. Then you have to cut and fabricate the car's body from the sheet metal, extract the rubber from the trees, then use that to make the tires and all the seals on the car. Use the pile of copper to make all the wires, and use the leftover rubber(you did save the scraps didn't you) to make the insulation. Melt down the pile of glass to make the windshield, side and back windows, also the headlights and lights themselves. Then you need to extract the crude oil from the well to refine your own engine oil and gas. In the end, you have a car created to your exact specifications (if you know what the hell you're doing) that may or may not be any better than just buying a car off the lot."
Of course I should additionally mention that Gentoo provides awesome documentation for all the steps and most of the actual assembly work is done single-handedly by portage!