Great news for Miku fans, but others might want to steer clear. Warning: if you can't read Japanese alright, don't buy.
Judging this game as fairly as I can, I would advise that anyone who considers buying this game should carefully gauge how much they like the vocaloid Hatsune Miku. If the answer is along the lines of "she's okay..." or "some of her songs are catchy", then you should probably avoid this game.
The reason is simple: rather than being a highly in depth game, this product is closer to being a fanservice for Vocaloids (and especially Hatsune Miku). This is a rhythm game. Gameplay goes like this: you choose a song, then you press buttons with timing, supposedly matching the beat of a song. It's pretty basic and won't thrill anyone looking for something innovative. All during your button pressing Miku (or if you unlock another character, that Vocaloid will take her place) will begin preforming some cute dance animation in some manner appropriate to the song.
The real enjoyment of the game comes from her actual preformance, the unlockable costumes, and the picture gallery (which contains many very skilled works of art), rather than the musical button mashing. Though keep in mind that players unused to beat games may have a hard time until they have played the game enough to get used to it.
The game also has a song editor, which will allow the player to create a dance and gameplay to match any mp3 they have on their psp. However, keep in mind that the menus are mostly in heavy Japanese, so the buyer should take his reading skills into account before considering purchase. Some portions of the game will play just fine without a lot of knowledge, but using the game's play editor will be nigh impossible with the proper skills.
Overall, this game is likely a waste of money if you fall into one of these categories:
1) Miku isn't particularly interesting to you.
2) You can't read Japanese well.
3) You would rather play a solid game than be able to watch Miku cutely prance around.