Dear God, I just spent three straight hours playing this game. This is easily the most addictive and fun game I've ever played. I'm playing it off the ISO, but I'm going into EBGames to trade in MGS Portable Ops and get the UMD tomorrow... it's THAT good!
There is a PSP, DS, and PC versions of the game. The PC demo is the same as the PSP. The AI is freaking brutal. I don't know what you guys mean by "cheap" but it plays damn good! It never misses an opportunity to f**k you over. I love it, it feels like I'm playing against an intelligent person. I've heard the companion glitch is meaningless, it's not a vital or important part of the game. From what I've seen on videos the DS version looks pretty crappy. Smaller screen, worse graphics, and the stylus is kind of difficult because you can accidently touch the wrong parts of the screen since it's so small.
Either way, no matter what version you buy, you need to have this game. Beats the SHIT out of Lumines.