The best time to change them is when you have between 1/8" and 1/4" left on the pad. If you let it get to the metail, it will be harder or impossible to turn the rotors.

I had Axxis PBR semi-mettalic pads and they were OUTSTANDING! They may have cost me about $40, but it was well worth the money - I also got some Brembo OEM style rotors for $29 each for the fronts. One day on the way home I had to stop quickly for a redlight and couldn't believe how smooth and fast it decelerated - I thought I was sliding!

EDIT: Most people think that squeaking generally means you need to replace the pads, but the #1 cause of squeaking is brake and other dust. If your pads are still in good health, try some anti-squeak from AutoZone - it's about $.99 for a package which will cover the front pads.