Speaking as a non-Catholic:
As an evangelical Anglican, I actually never use a rosary as an aid for prayer as this device is more in keeping with the Roman Catholic tradition, although I can very much see the use of such a device in prayer. The beads on the rosary represent various prayers and statements of faith, and, by 'saying' the rosary one can be sure that prayers are said without any being 'forgotten'.
Idolatry is the worship of anything that is not God - be it a golden calf, a statue, or even your car, holiday home in the Seychelles, or the Lottery. I cannot imagine any Catholic 'worshipping' the rosary as an object in itself; the rosary is simply an aid to prayer which some Christians find useful, and, therefore, using it cannot be classed as idolatry.
The only danger with the rosary is that, unless one makes a conscious effort to ensure that prayer comes from the heart, there is a temptation to 'go through the motions' in saying the rosary, so that prayer becomes mechanical and superficial. As long as the rosary is kept well in its place as an aid to personal, devotional and sincere prayer then there is nothing wrong with using it - and it is certainly not idolatrous - and, in fact, used in the right way it can enhance prayer a great deal.