The Final Fantasy series has many strengths, but only some of them have strong storylines while others have great gameplay (typically noted as the even number ones being better with the former and the odd numbers being better with the latter).

I don't believe I've commented on that many JRPGs, but it is very important, IMHO, to note that Final Fantasy XIII isn't really a JRPG - part of the streamlining process involved removing the very things that separate a JRPG from a Japanese made adventure/fighting game, which is why it's reception among Japaneses fans is so mixed.

So I think it only fair to note that this game is neither an RPG or a JRPG, and that the intent (which has been confirmed by multiple interviews with the producers and game designers) was to make a new type of game.

Hence if you are a fan of RPGS or JRPGs in particular, you may not like this game, and IMHO you should definitely rent it first.