How noticeable will the improvements be?
By being able to collect data 10x faster with ACS, conceivably, 10x the amount of discoveries can be made. In terms of image quality, it is akin to upgrading a digital camera. A difference will be noticable, but a person may have to know what to look for. Also, images taken with by ACS will not have the “missing” corner squares typical of the ones taken by Wide Field Planetary Camera 2.