I picked this up for twenty bucks today on lunch break, and it's awesome!
I remember playing Space Invaders on the old Commodore 64 back in the day, so I have fond memories of the franchise, but the core gameplay is pretty stale by now: Space Invaders is something you respect and admire on the condition that you don't actually have to play it. A friend whom I trust insisted I try it anyway, and I'm glad I did; this is an actual evolution of the Space Invader concept and I think it's a huge leap in the right direction. Think Pac Man Championship Edition and you aren't far off. The gameplay is basically the same as the original Space Invaders, but with all kinds of excellent power ups, bonus levels, and hidden stuff.
There are boss encounters where you have to shoot giant Space Invaders apart pixel by pixel. There is a laser power up that can obliterate entire blocks of enemies in seconds. There are UFO's that take you to hidden mini levels if you hit them. Basically, it's great.
It's twenty bucks new, I think it's on both PSP and DS (I picked up the DS version because I'm nerd enough to seriously consider importing the DS paddle attachment) and my local Gamestop had five or six of each version(much to my surprise)... I don't know why Square Enix would ship enough Space Invaders to go around but completely skimp on The World Ends With You, but that's another thread. The point is, it's available and cheap.
Taito really hit it out of the park with this one. Consider this one highly recommended.