Unless you've bought the character DLC from BBCS, the only new character introduced in EX is Relius. Otherwise, Makoto, Valkenhayn and Platinum would still be considered new for EX. It's up to you whether if you want to buy it or not since the properties of the characters's moves have changed a lot since then. Some combos which were from CS1 will still be present in CS:EX but they may either do more damage, or less. Combos that could originally be done mid-screen can only be done in corner now. The biggest change being almost every character's 3C (sweeping option) which makes it so that it is no longer possible to combo after except on counter hit if 3C is used as a starter. Another change happens to be the ability to use 2 gold bursts mid-combo increases the combo damage of the next chain by 130%, something that would have been useless in CS1. EX also has something called SMP (Same Move Proration). It's better you look this up than have me explain this to you. =]
The game offers a lot of rebalancing, toning characters up and down, some even stronger, some weaker.
CS1>CS2>CS:EX.
Characters more powerful in CS1 became weaker in CS2 then became powerful again in EX, but I really like the balancing done in EX.
I'll be getting CP when it comes out also, but I'm fine sticking with EX until then. I'm glad BlazBlue has a story and that's it's animated and uses 2D backgrounds+sprites which are what got me hooked onto the game. But in the end, it's still a fighting game, and fighting games aren't really meant to be played for the story. :P