If the DSi (which I know absolutely nothing about, other than the fact that it is the new version of the DS and sports a camera and a slightly larger screen) has the same homebrew abilities as the original DS and DS Lite, then it is definitely worth it. I think I have heard talk of concern for potential future Nintendo firmware updates on the DSi to block homebrew (unfortunately, the same devices that allow homebrew also allow very easy piracy of DS games :(

If you are on a budget, a regular DS or a DS Lite can be obtained very inexpensively these days (I use a "DS phat" as they have come to be called, paid $60 for it a few years back, bet they are really cheap now, and it works wonderfully and can do everything the Lite can do, save for the being able to adjust the screen brightness. The battery still lasts a good long while too, I go weeks at a time without charging it when under fairly heavy usage, months when all I use it for is light reading before bed.) A cheap DS or DS Lite and $20 more for a device to allow homebrew, and you've got yourself the swiss army knife of insomnia boredom fighters. The things are quite durable to, Nintendo still builds quality stuff, it is just too bad their native software is so mediocre.