I've been there on 24th Oct. The new terminal is brand new and .... I'd like to say unfinished. I'd prefer the old business class lounge though. Transfer process was bad. You need to go to a gate, which I can't remember, to board a coach to get there. Be aware that it may take at least 30 minutes because you will need to board a coach and have a new security check before entering into the new terminal.
The new terminal only serves GCC and Europe. My flight was from Bangkok to Frankfurt, which landed at the old terminal and left to Frankfurt from the new terminal. At that time, it seemed airport staffs didn't know how to transfer between these two terminals, you have to specifically ask Kuwait Airways staffs. On my way back, I ran around asking about three people to get from the new terminal to the old one, with only one hour connecting time. JFK is still served at the old terminal.