Nintendo has been on a winning streak since their original DS hit stores in 2004. That was followed by the redesigned DS Lite and then the insanely popular Wii.
The DSi, sporting two cameras, and the ability to download games, has quite a lot to live up to. But it's not such a radical departure from the formula that Nintendo stands much of a chance of it failing.
Perhaps the bigger question for Nintendo fans isn't if they should buy a DSi, but when.