[Mike]

As I understand, the Sharp sensors have a maximum range of 30 inches... 3.5 feet. When you have a robot moving 7 feet per second (conservative estimate), that means you have one half a second to "see" the object, make the calculations to find out which way to turn to avoid the object, and then actually make the motor adjustments to turn, and then actually turn. Ultrasonic sensors, however, can have a range substantially bigger. The SRF08 ($56.99) has a range from 1 inch to 18 feet.

This may work if you were to stop once you have detected an object, and then turn around your axis and continue forward, but that would be an awfully slow and inefficient way of doing things. This makes the sensors not very useful for object avoidance. It may be of good use for say, not ramming straight into a wall. This would be executed by mounting it on a servo, then once an object is detected, scan left and right to make sure the object is a wide flat area (and not say, a pole.) It would also be good for wall-following, but (at least in this years game) you would follow the wall for about 2 seconds and then ram straight into a side goal.

It all comes down to what you are trying to accomplish. When you are playing on a pre-determined surface (such as the FIRST field), I find an encoder/gyro combination to be a lot better. If you are in unfamiliar territory, then an ultrasonic/infrared sensor will be a better choice.