After three degrees (one in math, two in mech. eng.... and working on that PhD), TAing calculus I and II for one year and teaching them for another.... I'll give the standard engineering answer: it depends. <grin> The best thing I can say is that ANYONE with sufficient effort can plug through the first few courses with sheer determination. Calculus is one of those subjects that seems to "click" with some students and not with others (and not very predictably among "average" students, either). Some students LOVE it and see its implications... some struggle to keep your head above water.
But in short, try hard, and you'll get it. If you're a math theory person--its all going to begin for you next year. If you're a physics/engineering person (like myself), a little intuition (and hopefully a little instruction) will show you how the world works.
And for some of you that replied to this message: Undergraduate math (esp. that aimed at engineers and physicists) is remarkably simple. Hell, graduate level math aimed at engineers and physicists is simple. Most of it relies on "solve a bunch of problems just like the teacher did on the board". With a little effort, you can plug through it. Good luck to all.