I tried them. I saw them at Wal-Mart and decided to give them a try. They are about the size of Tums tablet (don't confuse the two!).
I put it in with my detergent at the start of the wash cycle. The tablet slowly disolves, and it has that "clorine" odor. I have found that for a large load of whites, it requires two tablets to do the same job as a cup of liquid bleach.
Overall, I don't think they are as effective as the liquid unless you let the clothes soak a little extra time. If you right into your wash cycle, I think the whites are brighter with liquid bleach. However, they sure are easy to use.