I've had a DS for ages now, but never used it all that much.
Then I figured out the homebrew/custom stuff and now I have a flashcart, with a 1gig microSD in it, that I'm using in my DS. I have any chat client I want, web browsing, etc.. can watch movies, play mp3's, play any game I want..
It's pretty much just brought the thing to life and then some, not to mention all the crazy shit the homebrew communities keep cooking up for it. Frigging insane.
I mean seriously. You can almost emulate any older handheld, or gaming system on it, play all THEIR games on it.. um, even all those old Lucasarts/etc adventure games.. if you want it to do something, it's probably already been coded.
Plus real time save and a bunch of other features now built into the chip you use is a godsend.