When your circular saw blade becomes dull, not only will it give you a rougher finish, but your saw's motor will have to work harder and may burn out sooner. Sharpening your saw blades will save you money in the long run because you'll get better cuts, and you'll have to replace your saw and blades less often.
You can sharpen steel-toothed circular saw blades yourself using a file. Carbide-tipped blades need to be taken to a professional sharpening service for two reasons: Carbide is so hard that you need a diamond wheel sharpener for it, and sharpening carbide blades is so complicated that you can easily ruin the blade. Carbide blades cut not only with the tips of the teeth but also with the sides, and if you're not careful, you can end up rounding off these edges. A sharpening service will return your saw blades to like-new condition for about 25 to 50 cents per tooth. You should ask a professional about other types of blades because some cannot be re-sharpened, and some blades are so cheap it might not even be worth the expense to get them sharpened. But don't throw them out -- save them for demolition work or cutting up lumber that might have nails hidden in it.
You can use a hand file, crank sharpener or automatic sharpener on saw blades you're going to sharpen yourself. While automatic sharpeners will get the job done faster, they can also be rough on your blade's teeth. They might overheat or lose the correct angle of the teeth they are sharpening. Although it requires more of your time and effort, filing your blades manually usually produces a better result.