Tens of millions of DSi’s predecessor, the DS Lite, have sold worldwide since its release in early 2006, according to Nintendo. In the meantime, the company anticipates that DSi unit sales have already reached an estimated 100 million worldwide, according to CNET.

Available in both blue and black, its retail price is set at $169.99. And along with the DSi’s release also comes a game new to U.S. gamers, “Rhythm Heaven.”

The DSi offers a longer stylus and larger screens, now 3.25 inches each. But in favor of a slimmer body, Nintendo removed the Game Boy Advance slot and replaced it with a SD card slot – a notable omission for anyone who, for example, just purchased “Guitar Hero: On Tour.”

Nintendo also added not just one but two cameras, with one fixed on the external body and the other pointed toward the user when open. This, as one company representative said to Go Nintendo, is part of the reason behind the ‘i’ in DSi, as it gives the gaming unit “eyes.”

“If the touch screen gave Nintendo DS a sense of feel and the microphone allowed it to hear, the two cameras gave Nintendo DSi the sense of sight,” the company explained in a press release last week.

These cameras have 11 different interactive “lenses,” each of which manipulate the photos in different ways. One, the Emoter lens, can create new facial expressions on the photographed person. Another, the Color lens, manipulates the colors of specific areas.

What also expands upon that personal aspect, or yet another reason behind the ‘i’ in DSi, is the enhanced sound application, featuring both a music player and voice recorder. Users can even manipulate the pitch and speed of their voice recordings, and save up to 18 ten-second clips onto the gaming system.

But perhaps its crowning feature will be its online DSi Shop, where users will be able to download games through a Wi-Fi connection. Nintendo is also encouraging developers in the meantime to create software through its DSi Ware application.

Nintendo hopes that all of these features add up to another indispensible product, that “becomes more than a game system and more of a personal tool to enrich our daily lives,” a company representative said.

But technology reviewers wonder if the upgrade is worth purchasing. Jeff Backalay, CNET reviewer, said that players with the first edition DS should consider the DSi. But to those with a DS Lite, “are all of these features enough to warrant an upgrade? In our opinion, the simple answer is no.”

On the other hand, Wired’s Chris Kohler anticipates that Nintendo’s push for digital distribution will become its main incentive.

“Downloading games will be awesome,” he wrote in his Nov. 17, 2008 preview.