I've just been saving up and buying 20 packs like a madman, feeding all the bronze and common players for teams I need to my created player and selling all the silvers, golds and diamonds to buy more packs.
I'm not a massive fan of using the FB cards tbh, I much prefer using the Live Series cards with their IE bonuses, so I'm mainly just opening packs for the fun of it, if I happen to get something cool like a rookie FB with missions or a great FB, then that's just a bonus. Probably the most useful thing is it's allowed me to complete about 20 or so of the feeding missions without any effort at all, while also helping to teach me for next year that opening packs really isn't an effective use of of my coins! It's been a lot of fun.