When you're working with fiber cement, straight cuts are the easiest to make, and a circular saw is the right tool for the job. There are a few things to keep in mind, though. First, you need to work on a sturdy worktable; sawhorses are not adequate for holding long, flimsy planks securely. Also, be sure to outfit your circular saw with blades made especially for the job. They produce less dust than regular blades, and they won't chew up the material. For large jobs, consider a purpose-built circular saw that attaches to a wet/dry vacuum.
Want to see more? Watch videos of Mike demonstrating how he makes cuts to fiber cement with hand tools and small power tools, and shears.
And to learn more about the tools used when working with fiber cement, read Fiber Cement Tools from Fine Homebuilding issue #193 (Feb/March 2008), pp.82-87.