Credit mega pack worth it?

If you really need the credits to stay competitive in the arena, then go for it as it will allow you to get the higher rank that yields more crystals (a return on your investment).

If you're buying it simply to level up additional characters for the Galactic Wars, then it varies on the individual and how many crystals you're getting daily from arena / dailies or how much money you're willing to spend on crystals.

I personally use 200 crystals daily to refresh the cantina once and my energy twice. Otherwise anything gained from the arena / dailies builds up as I refuse to buy chromium packs.