Welcome to the boards, RazorSmile! Always glad to have new blood.
Balancing Feruchemy is a concern we obviously have. I think the best weapon we have rest on 2 axes of the core "fuel" of the power - time and opportunity costs.
First, time. The books make notations throughout (particularly before the Seige) that Sazed has taken months to charge some of his feruchemical bracers, only to have those charges burn out in a few minutes or hours. This is a pretty big deal, in game time, because the cost should be such that a Feruchemist *always* questions whether or not it's worth burning off a charge before doing so - otherwise, the charge time's not high enough.
The second cost is opportunity - as in, what is the Feruchemist giving up for having these bursts of power later? My thoughts should be - quite a bit. Rather than a 1:1 ratio (in d20, for example, losing 1 point of Strength now for a point of Strength later), you could make it a 2:1, 3:1, or even 5:1 cost (depending upon the scale and the utility of the power being charged). The end result is charges are slower to accumulate and there's always something lost in the exchange. This means that the Feruchemist won't spend his time min-maxing every stat or ability to get the biggest advantage this second (and perhaps lose sight of the main game being played by everyone else), and will need to consider the opportunity costs of his choices before making them.
The best indicator of game balance is when there's no obvious "best choice" to make. This is something that's always driven our philosophy, and will continue to do so in Mistborn as well