I helped our primary school when they introduced it.
I'll start with the negatives, on lower levels they're aren't many book choices and so if you don't get the required score on the test to move levels you can get stuck.
Positives, when children were "free reading" no-one is checking that they have in fact read the book or that they understand it. Lots of children can read but cannot understand the story. AR checks they have read it and can understand it.
It makes children choose books they might not have picked up giving them wider genres, vocabulary, themes, characters (rather than reading every Horrid Henry book ever written.) This allows children to realise what they like and don't like, and like school, sometimes you have to read books that don't set your world on fire.
At the primary they have a 30 minute slot every day for AR and have ipads to do the tests on. Scores are monitored in school. They read once a week to either the teacher or LSA (what used to be called a TA) and they are asked questions so for younger readers, why do you think this person is sad? for older reader can you find the fronted adverbial sentence?
There is no real emphasis on point scoring or "winning" just a positive promotion of reading. The more you read, the better your vocabulary and your literacy skills increase. HTH.