Mercutio said:
Fully optimizes itself to your hardware, taking advantage of whatever unique improvements have been made with your CPU, motherboard chipset, memory controller... It's a half-step easier than doing a "roll your own" linux system and the results are supposed to be very impressive. I keep meaning to give it a try but I haven't been able to download it yet.
And this is the very reason it takes so long to intstall. Since it installs just about everything from source it has to compile just about everything. I understand KDE is a particularly big program.
Besides, the speed benefit Merc meantioned, the system used to install software is related to the system used in BSD. It is supposed to be much more reliable with regards to dependencies than other package systems.
Finally when it comes time to upgrade various parts of the system for security patches or increased functionality, other distributions will let you download the new version and install it and they'll even let you download the source for that package and compile it even including the same optimizations that Merc meantions. However, if you want to upgrade Gentoo you'll set the optimizations globally and then the the machine to upgrade (whatever the actual command is?) and it will download all the programs that are already on your machine and build all the programs with the optimizations you specified.
Once set in motion it takes longer and you can't use the system until it's finished but there is much less touching start to finish.