Yes, rent it. Then buy it, if you like it.

If you go into this expecting it to be just as good as whatever your favourite FF is, you'll likely be disappointed. If you go into it expecting it to be your new favourite FF, you probably haven't played very many. It's relatively weak compared to others in the series, but IMO, it's still a very good game. A worthy inclusion to anyone's 360 or PS3 library.

Where aspects of 'traditional' FF elements have been given less emphasis or omitted, you find a battle system that makes you think (as others have said, it's not just Attack, Attack, Attack. Even the average, non-boss monster can be demanding enough for you have to do a few paradigm shifts and work out a loose strategy). So there's no world map, towns, shops or end-game airship. It doesn't make it less of a game. A crappy battle system would - and FFXIII certainly doesn't have that.