We have one over the changetable and DD loves it! She can pull it down on top of her, but I'm there so it doesn't really matter.

As for the cot, my sister has one over her daughter's cot. When I put DD in there and leave the mobile up she doesn't sleep. She'll be quiet for ages just looking and playing and then eventually get upset. So I've started taking the mobile down and she settles.

Morale of the story - it may be too late to introduce your DS to something like this over his cot (e.g. he might view it as a toy rather than something soothing to help him sleep).