Personally I don’t compile Firefox (there’s a binary package) so my update times aren’t that bad. With LTO disabled for LLVM and any webkit package (qt one for kde, gtk one for gnome) if you use a desktoo environment, it’s usually fairy quick to install things. The initial build times when setting up the system can be several hours in total on midrange CPU though.

But if compiling all your packages for the ability to choose your own optimizations, package features (via USE flags) and just about everything else on your system doesn’t excite you, it’s probably not worth it. You can stick with Arch and still maintain pretty good control over your system compared to a more prebuilt distro. But if it does excite you, grab the Gentoo AMD64 handbook and get cracking! You can come over to !gentoo@lemm.ee with any questions (it’s not that active yet, but I do hope it’ll eventually be useful as a repository of knowledge and questions being asked and answered sure helps with that).