In it's first week in Japan, Final Fantasy XIII managed to sell over 1.5 million copies, and further more was able to push almost 250,000 PS3s. This means that about one third of all PS3 owners have Final Fantasy XIII. This number will probably increase over the next few weeks to over half.

While 1.5 million is not quite as high as the first week sales for Final Fantasy X and Final Fantasy XII, their launch was at a time when PS2 sales were much higher.

For example, FFX launched when the PS2 sales were at around 5.5 million, making it's launch attach rate around 34.5%. FFXIII on the other hand launched with PS3 sales at around 4.1 million (~4.3 million now that FFXIII has launched), which means it's attach rate is around 34.8, which is essentially the same as for FFX. Final Fantasy X had a huge drop in it's second week sales going from 1.9 million, to just 0.25 million. It will be interesting to see how much of a drop FFXIII has, considering the fewer PS3s sold in Japan right now, FFXIII could have pretty good legs down the road, and could eventually catch up to FFX and FFXII.
Source: Examiner