How much one would enjoy Hatsune Miku: Project DIVA is likely quite variable. I think very few people would dislike the game, but where you would end up on the scale from "yeah, it's fun" to "I freaking love this game" probably depends on 1) how much challenge you're looking for and 2) how interested you are in the Vocaloid phenomenon. If you regularly play DJ Max games and are only interested in that level of challenge, you may find DIVA too "casual." However, DIVA is extremely well presented and may still charm you even if you think it's too easy.
The entire package is a celebration of the user-generated Vocaloid content community: most of the songs are popular ones by skilled composers/manipulators of the Vocaloid vocal synthesis software, along with some original to the game; the load screens consist of an enormous amount of art created by numerous community artists; the game itself has an Edit Mode which lets you not only create button charts for any song in the game or on your Memory Stick (MP3), but even design and choreograph the music video for it starring Miku, and what you create can naturally be uploaded to the Internet for sharing (note that I don't know how to use the Edit Mode myself; I think there are fan-made English guides for it online, though).
Aside from the community portions, the game also just looks and sounds spectacular. Though some songs' visual portions consist of slideshows of static community art (all later viewable in the Visual Library Mode, along with all the load screens you have seen), the vast majority feature real-time 3D music videos with a model of Miku that has impressive motion capture animation and facial expressiveness. I'll admit, I don't like the Japanese idol-style dancing Miku does in some of the videos, but they're not all like that. The real-time bit comes into play because there are 50+ costumes ("modules"; among these are Alicia's uniform from Valkyria Chronicles and Ulala's outfit from Space Channel 5) unlockable via gameplay which you can switch to. Also, there is a ton of other unlockable objects to be used in the Miku Room mode; among these are a Sega Mark III, Master System, Mega Drive, and Game Gear. Miku Room is basically a little apartment you can decorate, and it's mostly good for taking screenshots of Miku doing a bunch of silly animations. You can also take screenshots anytime during gameplay and in the Visual Library Mode; instant PSP wallpapers!
HOWEVER, while playing the actual game is easily understood, the entire interface is in Japanese. Unless you can read Japanese, you may have problems navigating the interface and reading the unlockables' requirements (to say nothing of using the Edit Mode). I'm sure there are English guides on the Internet, though.
I myself mainly decided to get DIVA because of the Edit Mode. The flexibility it gave to essentially direct your own music videos (all the staging, camera placement/movement, dancing) was attractive. Ironically, I completely got sidetracked by playing the game and collecting all the stuff. The presentation is just so slick and fun. If that's enough for you to overlook the game not being obscenely difficult (except in a few songs' Hard versions), then I think you will enjoy it.
Incidentally, the sequel, Project DIVA 2nd, released last month in Japan, and it adds to the gameplay (D-pad keys, held-down buttons). Also, there is a DIVA arcade version with high-polycount models, and there is a 3000-yen PS3 application, Dreamy Theater, you can get from the PS Store which lets you connect the PSP to the PS3 and play the (original) DIVA on the TV in HD with high-polycount models. I'd like to get the PS3 thing someday; those may be the most lifelike anime-style renderings I've ever seen, and it's incredible that they're real-time graphics. Go, PS3!
How's that for off-topic?