The camera channel allows you to use one of the DSi's two cameras to snap photos of just about anything; the inward facing one of yourself, and the outward facing one of whatever poor soul you manage to use as a test subject. There's a handful of "real-time" filters you can apply to the photos adding things like frames, distortions, and even determining the similarity between two faces. I had some difficultly with the latter, I was only able to use it on my wife and daughter but could not get it to recognize my face and my daughter's at the same time. The photos are a minor amusement, ones that you can transfer back to your PC thanks to the built in SD card slot. A nice touch is the little calendar which shows you what day you took the pictures on - can you say time shift morph?
The sound channel lets you record your own sound files and distort and add filters to them. You can also playback AAC audio files from (unprotected) iTunes but you won't find any MP3 support here. During playback there's a handful of visualizers (like Excitebike!) and audio filter to apply in real time. It's a nice novelty, but not something you'd use every day for listening to music. I did have fun playing around with my voice and making it sing like a whole choir of me!
The screens themselves got a small update with a slightly larger size, it's one that without putting the system side by side with a DS Lite you probably won't notice. Still, in a game like GTA: Chinatown Wars I have to admit the characters seemed to pop off the screen a little bit more. It's probably more psychological than physical. The speakers are much improved, so much in fact, that in certain apps the "bass" of the system almost provides tactile feedback from pressing the on screen keyboard. The battery seems marginally better than my DS Lite through normal use, getting around 5 hours with heavy wi-fi use and about 12 hours without.
So is the DSi worth your hard earned dollar? I'd say if you haven't picked up a DS yet I'd go right for this version; otherwise you might need to make up an excuse to pick it up. Perhaps giving your significant other your old system so you can co-op one of the many co-op DS games together? It just might work. 
Score: 4 out of 5