The food at California Grill is very very good. One of the few restaurants that I would consider good, even away from Disney.
But to address OP's points--
It is most certainly NOT fine dining, not even close. It is a casual California fusion restaurant.
Whether a teenager would like it, depends on their tastes. My 4 year old loves sushi, for example.
But the teriyaki filet would probably go down easy on a picky eater -- it's really just steak with a sweet BBQ sauce.
Now, like all Disney restaurants, it's over priced.
I do think the view from CG is overrated -- you're going to have a great view from Poly club.
Portions are fairly large, as with all Disney restaurants. 2 moderate eaters could fairly share most of the entrees.
As to the
ddp, from a pure $$ value perspective, signatures aren't the best use of credits.
But if you got the ddp for free (so not looking to squeeze every $ out of every credit), and you are staying at club (where you can get apps before dinner), CG may be worth 2 credits.