2.8 comes with a port of Dream Drop Distance (the game that came out on 3DS), a brand new prologue to Kingdom Hearts 3 (thats the one thats very short) as well as a cinematic movie (not sure how long it is or where it takes place on the timeline). In my opinion, it's not worth full price for one full game, a demo of sorts, and a movie. I would personally wait until it hits the $30-40 range.
Now in your case, if you haven't played any of the games in a long time, then you must absolutely wait until March for the 1.5+2.5 collection. It's only going to sell for $50 and includes every single game up to the 2.8 game. That's 1.5 (basically the directors cut of KH 1), Chain of Memories (it was a gameboy title that they ported, the gameplay takes some time to get used to and not everyone loves it but it is crucial to the story so play it), the cinematics from another handheld, 2.5 (the directors cut of KH 2), another handheld spin off port, and another cinematic port of the final spin off. All that for $50, even if you have played them that is a steal.
