I took the ACTs a couple years ago. I liked it more (and did better on it) than the SATs. It's hard to tell you what to expect, and it's hard to prepare for. I felt like the ACT covered more information. There was a science section and such, which I thought was more interesting than the SATs. As I recall, it was interpretive rather than knowledge-based though, so don't worry if it's not a really strong suit.
Remember your calculator (one of my friends didn't). Get lots of sleep, eat right, relax, and good luck.
Why do we play with fire?
Why do we run our fingers through the flame?
Why do we leave our hands on the stove, although we know we're in for some pain?
-tick...tick...BOOM!