Per my opinion.
DO NOT use cartel coins to buy packs, it is not worth the money.
Things worth buying would be about half the things in your legacy panel, including rocket boost and xp boost increase, etc. Things that are only available on the Cartel Market?! I would say not much. Some gear, only when it's on sale (same goes for weapons), (character customization options, if you like different hairstyles, inventory slots. Not much.
Also, don't forget, you can always save your CC and spend it later when you rack up some coins!