Well, Gentoo's cool but not enough to pull me from Slack. I did a stage1 (or whichever's from the ground up) and might have had a screwed disk because it didn't work but it only took about five minutes to figure that out and download another. (Well, the downloading took longer.) Then it went without a hitch - just read the docs and follow along. Check the Gentoo site on USE flags and whatnot. Installs a base. Do a lot of emerging. Emerge '-p' (I think it is) is your friend. I emerged w3m for my first app and ended up with all of the X window system. And it's a damn console browser. Flipside is, it's that easy to put X on it.

Emerge generally works like a charm. Just keep your tree synced. However, I had an emerge fail and had no idea how to fix it when syncing didn't work. Just didn't feel like investing the time in learning a Gentoo-specific tool like that. Better to spend time directly on gcc and all.
So, yeah - read the docs, check out the USE flags, follow instructions, keep synced, emerge -p, get bored, come back to Slack.
-- Oh. Seriously - if you want to dual boot Slack and Gentoo ignore the stuff about devoting your whole freaking disk to Gentoo and installing GRUB and all. Just install it and get it to boot with LILO from Slack. I've got four distros on this thing and always just redo LILO from Slack.