If you're a total diehard fan of Kingdom Hearts, you desperately want to fight Xemnas in KH1, you want to try out the new abilities and Keyblades it has to offer, then yes. Get it. Otherwise, don't. It's practically just KH1 with cool extra features.
i own it & my opinion would be that you should buy it. That is of course if you don't mind reading all the Japanese. (it took me a wee bit to understand what was what, i.e. items. but it easy afterwards.)