The Nintendo DS has already experienced 2 major transformations and I figured I'd make a review to give you insight on each of them. Its a popular game which is helping to widen the gaming audience as I personally see more female gamers getting hooked by it. Its a family friendly system with plenty of child-safe fun titles that have withstood the test of time like Mario, Yoshi, MegaMan, Kingdom Hearts, Zelda and others.First I'll review the original DS. This was the bulkier model however it was built strong and is the best bet for young children who tend to drop things. You can probably buy it for around $50 used. It can be used as a calendar, alarm clock, and of course to play both DS and GBA games. It has bright LCD screens, the bottom being a touch screen for interactive DS games and you can use it with a WiFi signal in some games to play online with friends or random global competitors.
The DS Lite is a much smaller DS. Bigger, brighter screens and a new sleek design with all the same features as the DS. Definitely not great for kids as there are tons of reports of people having experience hinge breaks on the DS Lite. This system needs to be protected and I recommend the Nerf Armor to shockproof it. You can buy it for around $100.
The DSi is an amazing system. It has built in 1.3 megapixel cameras which actually shoot pretty good quality images which you can edit on the system. You can go online with a WiFi signal and do things like purchase games with Nintendo Points and download them directly onto the system instead of buying catridges. It doesn't play GBA games though. Its also got music playing capabilities, voice recording features, but right now there doesn't seem to be any DSi-exclusive games available yet in the US. Its going for like $170. You'll probably want the Nerf armor for this one too if its for a child to use.
One other problem is that Action Replay, a cheating device for the DS and DSLite can be used for online play. When you challenge random online players, often times someone will be hacking with AR to have invincibility or create an advantage that you can't surpass. So watch out for cheaters.
One major flaw though with online play is a lot of people may have difficulty setting up a WiFi signal. You can try the Nintendo site for their detailed troubleshooting guides on it but even then friends of mine still get a bit confused.
Update On Aug 17, 2009: The DSi is also compatible with SD cards and includes even brighter screen settings than the DS Lite. Larger screens as well.
One other cool thing with the DS for Wii owners is that you can go online on Wii through WiFi and download demos of DS games which you can download and play right on the system for free!
Update On Aug 17, 2009: It also appears like players can get exclusive add-ons to their games. I just checked the Wii WiFi section for DS download and play titles and there was a special battle card available for anyone who owned MegMan Starforce 3 for the DS. Its pretty cool and I assume there'll be more events like it for future games.