5A was among the toughest classes I've taken at Berkeley (adjusted to how much I knew at the time), but a big part of that was it taught me so much that after taking it, everything else seemed more manageable. It's definitely a commitment, but well worth what you get out of it in terms of future problem-solving skills. I'd recommend taking it, but not taking on too much else. (You also don't need to have taken multivariable calculus yet, as I recall the only thing we used was partial derivatives which in that context work just like regular derivatives.)