Ds13 participated from 5th through 8th and really enjoyed it, as a way to deepen his understanding of his favorite subject, math, and as a way to bond with other 'nerds' (their word). From my perspective it was extremely beneficial for him as it motivated him to work on getting over his fear of failure, and gave him what seems to me (a non-math person) a good grounding in discrete math topics.
Preparation: ds's school has a math club, the junior (4th-5th) version of which met once a week for an hour after school. The teacher/coach also made old AMC and MathCounts exams and practice MathCounts materials available to anyone who asked for them. In 5th ds probably spent a couple of hours a week on problem-solving work at home in addition to attending the club meetings. I don't remember how he did back then, but I know he feels now that the years of work are really paying off - this spring he qualified for AIME as a 12-year-old 8th grader.
Hope this helps -
