The first thing to be said about the DSi is that it is not a new handheld console. It is more of an upgrade to the Ds Lite. There are no new games made for the Dsi and there aren't a huge amount of new software updates, but the ones that there are, are quite fun.
There are three main additions to the DSi that aren't on the DS lite. There is the DSi shop, The internet browser and of course the camera.
First of all, I'll start with the DSi shop, the is very similar to the wii shop channel, but without the virtual console, which personally I think is the best bit about the wii shop channel, but there are still some good downloadable games. (I recommend Pyoro, and its a fun, simple, addictive game).
Next, the internet browser, which is poor to say the least. Like the Wii browser, it is powered by Opera which is normally a good web browser, in fact it's the one I'm using non, but on the DSi, it's just not very good, it's slow and it doesn't support flash, so don't bother using it to quickly check something up, because it would just be quicker to set up your laptop.
The last thing is the camera. The quality of the pictures it takes are average, but some of the things you can do with the pictures are quite cool. You can create your own frames and you can even change the emotion on your face by just pressing a button.
On last thing to be said about the DSi is the sound editor where you can change how your voice sounds and you can play your sound clips in reverse, which can be quite fun.
In conclusion, if you can't afford a 3DS and want a normal DS I would get this if you can pick it up for a good price.