I own this for the PSP. Great game for the PSP, but I will be eager to see if they add anything for the PS2 release.
Honestly, if they don't add anything and just release a port, then I think a lot of people will be disappointed. If you just play the missions, the game is really short! The stat log on San Andreas says that I played 109 hours of GTA before I beat the last mission and put the game on the shelf. It only took me 22 hours for GTA: Liberty City Stories to do the same thing. I completely understand the $19.99 price tag becaues there isn't as much content compared to the other GTA games on the PS2. I also don't feel ripped off for paying $49.99 for the game on the PSP because Liberty City Stories was made to be played on the portable system. While on the go, I could forgive the short length of the missions, the smaller amount of radio chatter and how Liberty City just doesn't seem as big after played San Andreas. It was/is great fun on the PSP.
So anyways, I like the game. I think it is worth $19.99 for anyone who likes the GTA world (The game gives some fun insight to the world of Grand Theft Auto... a lot of how things came to be type stuff.) I just hope that people know that this game isn't content-filled like San Andreas.. your return trip to Liberty City will be short.
Side Note: I haven't been able to play any of the multiplayer modes on the PSP (mostly because I don't know a lot of people with PSPs and the ones that do.. don't care for GTA.) I heard they are fun. It would be nice if you could play the multiplayer modes over the internet on the PS2... that would definitely help sell the game more IMHO.