I think AR was good for my kids early-on (maybe until about 4th grade). It made them read or they didn't make their goals. After 3rd grade, their reading levels were too high, so that the books were either gigantic (and they'd forget a lot by the time they finished & tried to test), old-fashioned/difficult to read,inappropriate, etc. It took the joy out of reading. Then the goals became drudgery. One of the boys favorite thing about going to the charter middle school is that they DON'T have AR. The public schools here have AR through 8th grade.
Cym, I found that to be a problem when selecting books at the top end of the reading level. But by searching for books at the desired level, but lower points, it gives results of books that are much shorter and quicker reads. So, GS would read 10 books at level 7 worth 1 point each instead of 1 book at level 7 worth 10 points. The result was same level, same total points. It got him through a spell where he would glance at a long book and whine, "It's too hard". Now he doesn't give a thought to the length.
