Finally unlocked all the songs in Project DIVA. Overall, it seems to be a fairly decent rythmn game, but it has a few flaws that become rather annoying after getting into it a bit.
The most notable problem is that the game occasionally lags. The framerate seems to drop on some songs, making hitting the beats rather difficult. I also found that occasionally, if you jostle the PSP accidentally, it causes the UMD to skip a bit, and the song and the beat you have to hit get desynchronized, basically forcing you to restart the song. (This seems to only happen near the start of songs, at least, perhaps it gets buffered up nicely later on?)
The songs in general are not as difficult as in some other games (Elite Beat Agents!). Getting a "Great" rank on normal difficulty is, for the most part, very straightforward, and easily attainable within three attempts at each song. One oddity seems to be that normal difficulty only uses the circle and x buttons -- easy difficulty uses the square and triangle occasionally, making me wonder whether this was an accident on part of the designers.
Perhaps the most interesting part is the Rythmn Edit Mode, which lets you basically make PVs and levels out of MP3s that you stick on your flash card. It's a neat feature that doesn't seem to show up often in such games. The saved levels can be shared with other people.
The selection of unlockable costumes for Miku is pretty extensive, though the last few seem to be recolored versions of each other. The other notable Vocaloids (Rin/Len, Luka, Kaito, Meiko, and Neru/Haku) can be unlocked as well, though with the exception of four songs, the voices and dance moves all remain Miku's regardless of who is picked. Watching Kaito do Miku's songs and moves is entertaining in a somewhat brain-melting way.
Songs are generally all fairly well known Miku works, by the expected artists (ryo, Oster Project, kz, etc.), there aren't any original works in the set, so anyone familiar with the Vocaloids would probably have heard most of them. The PVs are generally replaced by either the dancing CG Miku, or by slideshows with art taken from various sources.
Overall, the game is fairly straightforward, and it's hard to complain about a cute dancing Miku.