Good question that ... I just asked myself whether I would buy a PS3 for GT5 right now, given I wouldn't have one already. Well, let me say it like this: GT5 is the best and the worst Gran Turismo title yet. The best because it has the best graphics, the most cars and the most realistic handling yet. On the other hand, it is the one with the largest amount of mistakes, faults and bad decisions by PD yet. The driving itself is good fun and very realistic for a console game, but the career structure and several details of the game have some serious issues. Nothing that puts me off the game completely, but there are a lot of frustrating foibles.
Given you are into simulation-like driving games and you are ready to spend some serious time on a game to work your long way up from a Civic to a prototype racing monster, you should seriously consider buying a PS3 and GT5. If you are after casual drive now and then however, this game does not really warrant getting a PS3 for it, given you don't have any other use for the console.