IF youre on about the prologue then it depends if you got other games you like to play...This game is good I have to tell you, all licenses , 50 cars (that is alot but small relative to gt4) and 5 tracks where you can race the AI and the Italian track and some other ones here are great!
My result if you see it as a game
Graphics
8
Audio
8 (great car sounds)
Life
4 (if alone) 9 (With friends)
Its great to get if you want to practice but not to see it as a seperate game.
EDIT
And I dont think you will be able to use your saves from the prologue in the GT4 EU/US one, as I couldnt use my save from the GT3 US onto the GT3 EU