As someone who's not really a long-term Sales-Age expert, but does enjoy trying to get a better understanding of the business-side, I personally think getting an accurate idea of how the 3DS is performing to be one of the most difficult nuts to crack. My understanding of it is as follows: it got off to a rough start, but the price cut/ambassador program/big Mario games more or less salvaged it. As is, it's more or less doing "OK," but is underperforming Nintendo's expectations and is only truly soaring in Japan.
That's how I understand the situation. Basically, the overall assessment to me seems rather mediocre thus far. But there's no shortage of talk in terms of assessing it at the extremes that really muddies the waters for me. Some seem to classify it almost as an abject failure, while others just focus on the "tens of millions" sold to conclude that it's well on its way to being a very respectable successor to the smash hit DS.