I used to use a circular saw for cutting into aluminum siding when putting in new windows. It takes a steady hand. The biggest problem is usually you are on some sort of ladder while doing this. Don't overreach with the saw. Cut what you can reach without stretching, then move up and start again. Blade depth shallow as others have mentioned.
I now use a saw I bought at sears a number of years ago on a whim. It has two blades that counter rotate against each other. The motor looks like an angle grinder. It leaves a very clean cut and is a lot of easier to control. Good eye protection is a must. The aluminum chips that come off fly everywhere and they are on the large side. Wear some thin gloves too. I have a bunch of pairs of craftsmen work gloves. The cut siding is sharp.