The battery life is relative - you can set the screen super bright and wear the battery down quickly, or you can keep it fairly dark and the battery will last up to 14 hrs (although that is very dark). It's very easy to adjust brightness without exiting your game, just hold Select and use the volume control to go up or down. I like that you can make it really bright when you're in the car and there's too much glare, or turn it way down when in a dark room.
I really didn't think the bigger/brighter screen would look any different but I love it, my old games look brand new again! My screen protector on the DS Lite was pretty scratched up, but still. I'm seeing background details I never noticed before!
The camera is little more than a toy, it's cute but I probably won't use it much (although I like seeing my personal pics pop up when I turn on the machine). Same for the music player feature, that's what my iPod is for. I like the nicer speakers though! They do sound a LOT better.
I never surfed the web on my DS Lite, and I don't have a smartphone, so I'm really blown away by the internet capabilities of the DSi. The browser is free, and it can load more sites than I expected. No flash player, but still - I can check my email, the weather, play sudoku, twitter, shop, etc. The interface is really easy & intuitive and I seriously can't put it down.
There isn't much DSiWare out there yet, true. You get 1000 free points for shopping if you buy the DSi before July (you don't have to use the points by July though, you can wait) so that's like $10 free. I'm not so critical of game quality when they're free, lol. But so far I got Aquia and it is a fun puzzler.
I traded in my old DS Lite at Gamestop for $70, plus brought in some old Wii and DS games so I wound up spending around $75 total. I'm VERY glad I did, I love it!!!