I used one to buy a few minutes to grab a shower or something. It doesn’t last long, but it’s useful and it never caused sleep issues for us. Probably didn’t use it more than a couple times a week, fwiw.

Get a good interesting one. Think of it as a Calder for the crib. My DCs loved them - they would look at them and “talk” at them when they were settling down.

Weissbluth says bad fwiw

One baby loved mobiles. It would buy us 30 minutes to take a shower or whatever.

We never used one in the crib, figuring it would be too distracting. Ours was a good sleeper as a baby, and we didn't want to mess with that.

Instead we had a play mat with (optional) soft arches above, with a black-and-white mobile hanging down. The high contrast is easier for newborns to see. They get the stimulation of the mobile, but it doesn't disrupt sleep.

what should a mobile be used for? to help put baby to sleep? that doesnt really create good sleeping habits? also is it bad to let baby play with mobile awake fully? then he doesnt associate his crib with sleep...?