Linux From Scratch may be one of the greatest software adventures of all time.
Sure, if an "adventure" is what you're looking for. Gentoo is probably better if you want an adventure, but actually want to use the computer for something useful afterward...or if you want to start doing normal desktop stuff immediately, if the idea of compiling a kernel scares you, just go for Ubuntu. We wouldn't want you (anyone) to judge Linux poorly if you fail to get a LFS system working smoothly...
True but to learn linux, I mean gentoo teaches portage, ubuntu teaches apt or maybe yum something, even if they do wind up downloading a fedora 7 dvd in frustration afterwards, they will have learned linux. Learning X is something else.
No distro is entirely, or even mostly, its package manager. Gentoo is a lot more than that - you learn about all aspects of Linux, maybe not in as much depth as LFS, but you end up with a good system (generally) and it's easy to get help, and there's a great installation guide. Just using Ubuntu doesn't teach that much about it, but it's great for a general-use PC that you want to get up and running quickly. I don't think it really teaches much about apt, beyond how to do "apt-get install xxx," but it's easy to use. I still recommend Gentoo for learning, because you can get a well-configured PC out of it with a manageable amount of time and effort, and still learn all about the system.