I "tried" its psp iteration once, the one that was "somewhat fanlated" and I found it HARD! I'd buy this new game in a heart beat if I understood japanese, sadly, I dont so, no.

Well, vocaloid/hatsune miku for some reason does have quite a big following within the japanophile community. I never played any of the games, but it's a rythm game with strange/cute background videos and computergenerated voices, right ? I think I'll stay with rock band/ddr.

If you played Extend it's normal, it's the hardest by far. Generally speaking Project Diva games are hard if you're not a Hastune Miku fan to begin with. The reason for this is that if you're familiar with a song (i.e. if you are fan enough to listen to it repeatly when commuting, as I do) you brain already knows the tempo and you can focus on the buttons. You don't even try a song in hard mode before you know it by heart. Basically the normal mode in Extend is equivalent to the hard mode in 2nd or F, a the extreme mode is... eh I don't know what it is for it's so fu*** hard! And you don't need to know Japanese to play it : there's only the menu, and countless fan translations already exist (there's even one in French). 

Btw I noticed something really interesting : PSVita sales are up AGAIN this week in Europe while hardware in general is down and Vita software is also down. Last week Vita surged in France (+34%), it exploded in Germany (+57%) and was up slightly in Britain (+5%). This week Vita sales grew again in Britain (+11%), increased further in Germany (+18%) and were slightly down in France (-5%). We in Europe haven't seen a PSVita game for ages (and won't be seeing that many actually...) so Project Diva F has a small but sizable influence in countries it wasn't developed for at all! Such is the power of Hatsune Miku.

I love it, beat 20 songs on Hard already!

Where do you get these numbers?

Not very far from here : in VGchartz weekly charts, you have detailed info for France, Germany and UK (hardware sales are on the right)