There's really not much difference when you get down the end result.
Portage is going to obviously be slower since you're dealing with source, but you also have more control of things, as an example, using flags to compile to a specific architecture or what dependencies will be installed.
The biggest pains of portage IMO, mask and umask items and how incredibly easy it is to shoot yourself in the foot and break things because you just went off and updated the world or something
I know I've hosed gentoo a couple of times when I used it and trust me, it's easy to do...but hey, you learn from mistakes right?
I don't want to say that rpm and deb is any better, but it is quicker and typically "always works" as they're a little more static then what portage would be.
As far as usage, just think of emerge like yum or apt-get. Gentoo, does have a pretty good documentation system, so you should be able to find most of your answer there.