[onewhippedpuppy]

Don't use a sawzall, it will chip the hell out of it.

A circular saw with a good sharp fine toothed blade or a sabre saw with a good blade will do it. To help prevent chipping put masking tape over the cutlines, mark on top of the masking tape, and then cut, it will help limit any chipping.

If you are cutting an already made top it can be a challenge unless you have a good tablesaw with a big table. I've done it with a circular saw per the above and it has come out ok but not perfect. Mitre saw would work, but most don't have the reach to cut the width of a countertop, even sliding saws. If your cuts are going to be exposed or will be a seam between two pieces of countertop, it will be very difficult to get a straight enough cut that makes for a clean seam. Ideally your cut end would butt up against a cabinet or wall and you could hide the seam with caulking, trim, tile, etc.

Maybe you could take it to a local lumberyard and pay them a little bit to cut it? They typically have a tablesaw or radial arm saw that would be up to the task.