Condoms are 99% percent safe... when used PROPERLY
An important factor to consider is proper storage - extreme temperatures (below 40F or above 100F) can damage the latex of the condom, causing it do develop microscopic holes. These holes can be big enough for a sperm (or an AIDS virus) to pass through, but too small for you to see. In other words, you can't rely on sight to verify that a condom is OK.
So, buy your own condoms. Get them at the store, not at the free clinic, and check the expiration date. Store them somewhere at room temperature (not in your wallet or your dashboard). Make sure to put it on right - you have to pinch the end to create some slack so that the pressure from cumming in it doesn't cause it to break - and when your done, hold the end of the condom on you so that it doesn't slide off when you pull out (this has happened to friends of mine before - you don't want to be putting a hand up there to fish it out).
So, follow all the proper precedures and you'll be 98 or 99% safe. In general, though, the effectiveness of condoms is much lower because people don't use them right.